FESTO CORP. v. SHOKETSU KINZOKU KOGYO KABUSHIKI CO., LTD., et al.

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Syllabus
FESTO CORP. v. SHOKETSU KINZOKU KOGYO
KABUSHIKI CO., LTD., et al.
certiorari to the united states court of appeals for
the federal circuit
No. 00–1543. Argued January 8, 2002—Decided May 28, 2002
Petitioner Festo Corporation owns two patents for an industrial device.
When the patent examiner rejected the initial application for the first
patent because of defects in description, 35 U. S. C. § 112, the application
was amended to add the new limitations that the device would contain
a pair of one-way sealing rings and that its outer sleeve would be made
of a magnetizable material. The second patent was also amended dur-
ing a reexamination proceeding to add the sealing rings limitation.
After Festo began selling its device, respondents (hereinafter SMC) en-
tered the market with a similar device that uses one two-way sealing
ring and a nonmagnetizable sleeve. Festo filed suit, claiming that
SMC’s device is so similar that it infringes Festo’s patents under the
doctrine of equivalents. The District Court ruled for Festo, rejecting
SMC’s argument that the prosecution history estopped Festo from say-
ing that SMC’s device is equivalent. A Federal Circuit panel initially
affirmed, but this Court granted certiorari, vacated, and remanded in
light of Warner-Jenkinson Co. v. Hilton Davis Chemical Co., 520 U. S.
17, 29, which had acknowledged that competitors may rely on the prose-
cution history to estop the patentee from recapturing subject matter
surrendered by amendment as a condition of obtaining the patent. On
remand, the en banc Federal Circuit reversed, holding that prosecution
history estoppel applied. The court ruled that estoppel arises from any
amendment that narrows a claim to comply with the Patent Act, not
only from amendments made to avoid the prior art, as the District Court
had held. The Federal Circuit also held that, when estoppel applies, it
bars any claim of equivalence for the element that was amended. The
court acknowledged that, under its prior cases, prosecution history es-
toppel constituted a flexible bar, foreclosing some, but not all, claims
of equivalence, depending on the purpose of the amendment and the
alterations in the text. However, the court overruled its precedents
on the ground that their case-by-case approach had proved unworkable.
Held: Prosecution history estoppel may apply to any claim amendment
made to satisfy the Patent Act’s requirements, not just to amendments
made to avoid the prior art, but estoppel need not bar suit against every
equivalent to the amended claim element. Pp. 730–742.

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(a) To enable a patent holder to know what he owns, and the public
to know what he does not, the inventor must describe his work in “full,
clear, concise, and exact terms.” § 112. However, patent claim lan-
guage may not describe with complete precision the range of an inven-
tion’s novelty. If patents were always interpreted by their literal
terms, their value would be greatly diminished. Insubstantial substi-
tutes for certain elements could defeat the patent, and its value to inven-
tors could be destroyed by simple acts of copying. Thus, a patent’s
scope is not limited to its literal terms, but embraces all equivalents
to the claims described. See Winans v. Denmead, 15 How. 330, 347.
Nevertheless, because it may be difficult to determine what is, or is not,
an equivalent, competitors may be deterred from engaging in legitimate
manufactures outside the patent’s limits, or lulled into developing com-
peting products that the patent secures, thereby prompting wasteful
litigation. Each time the Court has considered the doctrine of equiva-
lents, it has acknowledged this uncertainty as the price of ensuring the
appropriate incentives for innovation, and it has affirmed the doctrine
over dissents that urged a more certain rule. See, e. g., id., at 343, 347.
Most recently, Warner-Jenkinson, supra, at 28, reaffirmed the doc-
trine. Pp. 730–733.
(b) Prosecution history estoppel requires that patent claims be inter-
preted in light of the proceedings before the Patent and Trademark
Office (PTO). When the patentee originally claimed the subject matter
alleged to infringe but then narrowed the claim in response to a rejec-
tion, he may not argue that the surrendered territory comprised an
unforeseen equivalent. See Exhibit Supply Co. v. Ace Patents Corp.,
315 U. S. 126, 136–137. The rejection indicates that the patent exam-
iner does not believe the original claim could be patented. While the
patentee has the right to appeal, his decision to forgo an appeal and
submit an amended claim is taken as a concession that the invention as
patented does not reach as far as the original claim. See, e. g., Good-
year Dental Vulcanite Co. v. Davis, 102 U. S. 222, 228. Were it other-
wise, the inventor might avoid the PTO’s gatekeeping role and seek to
recapture in an infringement action the very subject matter surren-
dered as a condition of receiving the patent. Pp. 733–735.
(c) Prosecution history estoppel is not limited to amendments in-
tended to narrow the patented invention’s subject matter, e. g., to avoid
prior art, but may apply to a narrowing amendment made to satisfy any
Patent Act requirement, including § 112’s requirements concerning the
patent application’s form. In Warner-Jenkinson, the Court made clear
that estoppel applies to amendments made for a “substantial reason re-
lated to patentability,” 520 U. S., at 33, but did not purport to catalog
every reason that might raise an estoppel. Indeed, it stated that even

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if the amendment’s purpose were unrelated to patentability, the court
might consider whether it was the kind of reason that nonetheless might
require estoppel. Id., at 40–41. Simply because estoppel has been dis-
cussed most often in the context of amendments made to avoid the prior
art, see, e. g., id., at 30, it does not follow that amendments made for
other purposes will not give rise to estoppel. Section 112 requires that
the application describe, enable, and set forth the best mode of carrying
out the invention. The patent should not issue if these requirements
are not satisfied, and an applicant’s failure to meet them could lead to
the issued patent being held invalid in later litigation. Festo’s argu-
ment that amendments made to comply with § 112 concern the applica-
tion’s form and not the invention’s subject matter conflates the patent-
ee’s reason for making the amendment with the impact the amendment
has on the subject matter. Estoppel arises when an amendment is
made to secure the patent and the amendment narrows the patent’s
scope. If a § 112 amendment is truly cosmetic, it would not narrow the
patent’s scope or raise an estoppel. But if a § 112 amendment is neces-
sary and narrows the patent’s scope—even if only for better descrip-
tion—estoppel may apply. Pp. 735–737.
(d) Prosecution history estoppel does not bar the inventor from as-
serting infringement against every equivalent to the narrowed element.
Though estoppel can bar challenges to a wide range of equivalents, its
reach requires an examination of the subject matter surrendered by
the narrowing amendment. The Federal Circuit’s complete bar rule is
inconsistent with the purpose of applying the estoppel in the first
place—to hold the inventor to the representations made during the ap-
plication process and the inferences that may be reasonably drawn from
the amendment. By amending the application, the inventor is deemed
to concede that the patent does not extend as far as the original claim,
not that the amended claim is so perfect in its description that no one
could devise an equivalent. The Court’s view is consistent with prece-
dent and PTO practice. The Court has consistently applied the doc-
trine in a flexible way, considering what equivalents were surrendered
during a patent’s prosecution, rather than imposing a complete bar that
resorts to the very literalism the equivalents rule is designed to over-
come. E. g., Goodyear Dental, supra, at 230. The Federal Circuit
ignored Warner-Jenkinson’s instruction that courts must be cautious
before adopting changes that disrupt the settled expectations of the
inventing community. See 520 U. S., at 28. Inventors who amended
their claims under the previous case law had no reason to believe they
were conceding all equivalents. Had they known, they might have
appealed the rejection instead. Warner-Jenkinson struck the appro-
priate balance by placing the burden on the patentee to prove that an

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Syllabus
amendment was not made for a reason that would give rise to estoppel.
Id., at 33. Similarly, the patentee should bear the burden of showing
that the amendment does not surrender the particular equivalent in
question. As the author of the claim language, his decision to narrow
his claims through amendment may be presumed to be a general dis-
claimer of the territory between the original claim and the amended
claim. Exhibit Supply, supra, at 136–137. However, in cases in which
the amendment cannot reasonably be viewed as surrendering a particu-
lar equivalent—e. g., where the equivalent was unforeseeable at the time
of the application or the rationale underlying the amendment bears but
a tangential relation to the equivalent—the patentee can rebut the pre-
sumption that prosecution history estoppel bars a finding of equivalence
by showing that at the time of the amendment one skilled in the art
could not reasonably be expected to have drafted a claim that would
have literally encompassed the alleged equivalent. Pp. 737–741.
(e) Whether Festo has rebutted the presumptions that estoppel
applies and that the equivalents at issue have been surrendered should
be determined in the first instance by further proceedings below.
Pp. 741–742.
234 F. 3d 558, vacated and remanded.
Kennedy, J., delivered the opinion for a unanimous Court.
Robert H. Bork argued the cause for petitioner. With him
on the briefs were Charles R. Hoffmann, Gerald T. Bodner,
Glenn T. Henneberger, Anthony E. Bennett, Andrew L. Frey,
Donald M. Falk, and Robert L. Bronston.
Deputy Solicitor General Wallace argued the cause for
the United States as amicus curiae. With him on the brief
were Solicitor General Olson, Acting Assistant Attorney
General Schiffer, Jeffrey P. Minear, Vito J. DiPietro, An-
thony J. Steinmeyer, Howard S. Scher, and Linda Moncys
Isacson.
Arthur I. Neustadt argued the cause for respondents.
With him on the brief were Charles L. Gholz, Robert T. Pous,
and James B. Lampert.*
*Briefs of amici curiae urging reversal were filed for the American Bar
Association by Robert E. Hirshon, E. Anthony Figg, Minaksi Bhatt, and
Robert H. Cameron; for the American Intellectual Property Law Asso-
ciation by Lawrence M. Sung and Janice M. Mueller; for ASTA Medica

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Opinion of the Court
Justice Kennedy delivered the opinion of the Court.
This case requires us to address once again the relation
between two patent law concepts, the doctrine of equivalents
and the rule of prosecution history estoppel. The Court
considered the same concepts in Warner-Jenkinson Co. v.
Hilton Davis Chemical Co., 520 U. S. 17 (1997), and reaf-
Aktiengesellschaft by Steven B. Kelber; for Bose Corp. by Charles Hieken
and Frank P. Porcelli; for Celltech Group plc. by Donald S. Chisum; for
Chiron Corp. by Mr. Chisum; for the Federal Circuit Bar Association by
Claire Laporte, Mitchell J. Matorin, and George E. Hutchinson; for Fe´ d-
e´ ration Internationale des Conseils en Proprie´ te´ Industrielle by Maxim
H. Waldbaum, Raymond C. Stewart, John P. Sutton, and Tipton D. Jen-
nings IV; for the Houston Intellectual Property Law Association by
Sharon A. Israel; for Intellectual Property Creators et al. by Steven L.
Winter; for Litton Systems, Inc., by John G. Roberts, Jr., Catherine E.
Stetson, Rory J. Radding, and Stanton T. Lawrence III; for the Minnesota
Mining and Manufacturing Co. et al. by Carter G. Phillips, Joseph R.
Guerra, Mark E. Haddad, Gary L. Griswold, Robert A. Armitage, Philip
S. Johnson, Wayne C. Jaeschke, Peter C. Richardson, and Kenneth Olson;
for the National Bar Association by Edward W. Gray, Jr., Kendrew H.
Colton, and John Moses; for the National Intellectual Property Law In-
stitute by James Phillip Chandler; for the Wisconsin Alumni Research
Foundation et al. by Susan G. Braden, Kevin M. O’Brien, and Michael E.
Murphy; and for Vincent P. Tassinari by Mr. Tassinari, pro se.
Briefs of amici curiae urging affirmance were filed for the Consumer
Project on Technology by Joshua D. Sarnoff; for Genentech, Inc., by Jef-
frey P. Kushan and Marinn F. Carlson; for Intel Corp. et al. by Terry E.
Fenzl, Alan H. Blankenheimer, and Howard Ross Cabot; and for Interna-
tional Business Machines Corp. et al. by Christopher A. Hughes, Mark J.
Abate, Frederick T. Boehm, Mark F. Chadurjian, William J. Coughlin,
Barry Estrin, and Richard Whiting.
Briefs of amici curiae were filed for Applera Corp. et al. by Matthew
D. Powers and Edward R. Reines; for the Institute of Electrical and Elec-
tronics Engineers-United States of America by Andrew C. Greenberg and
Matthew J. Conigliaro; for Medimmune, Inc., by Harvey Kurzweil and
Henry J. Ricardo; for the Patent, Trademark, & Copyright Section of the
Bar Association of the District of Columbia by William P. Atkins; for the
Philadelphia Intellectual Property Law Association by Joan Taft Kluger
and Manny D. Pokotilow; and for Sean Patrick Suiter by Mr. Suiter,
pro se.

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firmed that a patent protects its holder against efforts of
copyists to evade liability for infringement by making only
insubstantial changes to a patented invention. At the same
time, we appreciated that by extending protection beyond
the literal terms in a patent the doctrine of equivalents can
create substantial uncertainty about where the patent mo-
nopoly ends. Id., at 29. If the range of equivalents is un-
clear, competitors may be unable to determine what is a
permitted alternative to a patented invention and what is
an infringing equivalent.
To reduce the uncertainty, Warner-Jenkinson acknowl-
edged that competitors may rely on the prosecution history,
the public record of the patent proceedings. In some cases
the Patent and Trademark Office (PTO) may have rejected
an earlier version of the patent application on the ground
that a claim does not meet a statutory requirement for pat-
entability. 35 U. S. C. § 132 (1994 ed., Supp. V). When the
patentee responds to the rejection by narrowing his claims,
this prosecution history estops him from later arguing that
the subject matter covered by the original, broader claim
was nothing more than an equivalent. Competitors may
rely on the estoppel to ensure that their own devices will not
be found to infringe by equivalence.
In the decision now under review the Court of Appeals for
the Federal Circuit held that by narrowing a claim to obtain
a patent, the patentee surrenders all equivalents to the
amended claim element. Petitioner asserts this holding de-
parts from past precedent in two respects. First, it applies
estoppel to every amendment made to satisfy the require-
ments of the Patent Act and not just to amendments made
to avoid pre-emption by an earlier invention, i. e., the prior
art. Second, it holds that when estoppel arises, it bars suit
against every equivalent to the amended claim element.
The Court of Appeals acknowledged that this holding de-
parted from its own cases, which applied a flexible bar when
considering what claims of equivalence were estopped by the

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Opinion of the Court
prosecution history. Petitioner argues that by replacing the
flexible bar with a complete bar the Court of Appeals cast
doubt on many existing patents that were amended during
the application process when the law, as it then stood, did
not apply so rigorous a standard.
We granted certiorari to consider these questions.
I
Petitioner Festo Corporation owns two patents for an im-
proved magnetic rodless cylinder, a piston-driven device that
relies on magnets to move objects in a conveying system.
The device has many industrial uses and has been employed
in machinery as diverse as sewing equipment and the Thun-
der Mountain ride at Disney World. Although the precise
details of the cylinder’s operation are not essential here, the
prosecution history must be considered.
Petitioner’s patent applications, as often occurs, were
amended during the prosecution proceedings. The appli-
cation for the first patent, the Stoll Patent (U. S. Patent
No. 4,354,125), was amended after the patent examiner re-
jected the initial application because the exact method of
operation was unclear and some claims were made in an
impermissible way. (They were multiply dependent.) 35
U. S. C. § 112 (1994 ed.). The inventor, Dr. Stoll, submitted
a new application designed to meet the examiner’s objections
and also added certain references to prior art. 37 CFR
§ 1.56 (2000). The second patent, the Carroll Patent (U. S.
Patent No. 3,779,401), was also amended during a reexamina-
tion proceeding. The prior art references were added to
this amended application as well. Both amended patents
added a new limitation—that the inventions contain a pair of
sealing rings, each having a lip on one side, which would
prevent impurities from getting on the piston assembly.
The amended Stoll Patent added the further limitation that
the outer shell of the device, the sleeve, be made of a magne-
tizable material.

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After Festo began selling its rodless cylinder, respondents
(whom we refer to as SMC) entered the market with a device
similar, but not identical, to the ones disclosed by Festo’s
patents. SMC’s cylinder, rather than using two one-way
sealing rings, employs a single sealing ring with a two-way
lip. Furthermore, SMC’s sleeve is made of a nonmagnetiz-
able alloy. SMC’s device does not fall within the literal
claims of either patent, but petitioner contends that it is so
similar that it infringes under the doctrine of equivalents.
SMC contends that Festo is estopped from making this
argument because of the prosecution history of its patents.
The sealing rings and the magnetized alloy in the Festo
product were both disclosed for the first time in the amended
applications. In SMC’s view, these amendments narrowed
the earlier applications, surrendering alternatives that are
the very points of difference in the competing devices—the
sealing rings and the type of alloy used to make the sleeve.
As Festo narrowed its claims in these ways in order to obtain
the patents, says SMC, Festo is now estopped from saying
that these features are immaterial and that SMC’s device is
an equivalent of its own.
The United States District Court for the District of Massa-
chusetts disagreed. It held that Festo’s amendments were
not made to avoid prior art, and therefore the amendments
were not the kind that give rise to estoppel. A panel of the
Court of Appeals for the Federal Circuit affirmed. 72 F. 3d
857 (1995). We granted certiorari, vacated, and remanded
in light of our intervening decision in Warner-Jenkinson Co.
v. Hilton Davis Chemical Co., 520 U. S. 17 (1997). After a
decision by the original panel on remand, 172 F. 3d 1361
(1999), the Court of Appeals ordered rehearing en banc to
address questions that had divided its judges since our deci-
sion in Warner-Jenkinson. 187 F. 3d 1381 (1999).
The en banc court reversed, holding that prosecution his-
tory estoppel barred Festo from asserting that the accused
device infringed its patents under the doctrine of equiva-

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lents. 234 F. 3d 558 (2000). The court held, with only one
judge dissenting, that estoppel arises from any amendment
that narrows a claim to comply with the Patent Act, not only
from amendments made to avoid prior art. Id., at 566.
More controversial in the Court of Appeals was its further
holding: When estoppel applies, it stands as a complete bar
against any claim of equivalence for the element that was
amended. Id., at 574–575. The court acknowledged that
its own prior case law did not go so far. Previous decisions
had held that prosecution history estoppel constituted a flex-
ible bar, foreclosing some, but not all, claims of equivalence,
depending on the purpose of the amendment and the alter-
ations in the text. The court concluded, however, that its
precedents applying the flexible-bar rule should be overruled
because this case-by-case approach has proved unworkable.
In the court’s view a complete-bar rule, under which estoppel
bars all claims of equivalence to the narrowed element,
would promote certainty in the determination of infringe-
ment cases.
Four judges dissented from the decision to adopt a com-
plete bar. Id., at 562. In four separate opinions, the dis-
senters argued that the majority’s decision to overrule prec-
edent was contrary to Warner-Jenkinson and would unsettle
the expectations of many existing patentees. Judge Michel,
in his dissent, described in detail how the complete bar re-
quired the Court of Appeals to disregard 8 older decisions of
this Court, as well as more than 50 of its own cases. 234
F. 3d, at 601–616.
We granted certiorari. 533 U. S. 915 (2001).
II
The patent laws “promote the Progress of Science and use-
ful Arts” by rewarding innovation with a temporary monop-
oly. U. S. Const., Art. I, § 8, cl. 8. The monopoly is a prop-
erty right; and like any property right, its boundaries should
be clear. This clarity is essential to promote progress, be-

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cause it enables efficient investment in innovation. A patent
holder should know what he owns, and the public should
know what he does not. For this reason, the patent laws
require inventors to describe their work in “full, clear, con-
cise, and exact terms,” 35 U. S. C. § 112, as part of the deli-
cate balance the law attempts to maintain between inven-
tors, who rely on the promise of the law to bring the
invention forth, and the public, which should be encouraged
to pursue innovations, creations, and new ideas beyond the
inventor’s exclusive rights. Bonito Boats, Inc. v. Thunder
Craft Boats, Inc., 489 U. S. 141, 150 (1989).
Unfortunately, the nature of language makes it impossible
to capture the essence of a thing in a patent application.
The inventor who chooses to patent an invention and disclose
it to the public, rather than exploit it in secret, bears the risk
that others will devote their efforts toward exploiting the
limits of the patent’s language:
“An invention exists most importantly as a tangible
structure or a series of drawings. A verbal portrayal
is usually an afterthought written to satisfy the require-
ments of patent law. This conversion of machine to
words allows for unintended idea gaps which cannot be
satisfactorily filled. Often the invention is novel and
words do not exist to describe it. The dictionary does
not always keep abreast of the inventor. It cannot.
Things are not made for the sake of words, but words
for things.” Autogiro Co. of America v. United States,
384 F. 2d 391, 397 (Ct. Cl. 1967).
The language in the patent claims may not capture every
nuance of the invention or describe with complete precision
the range of its novelty. If patents were always interpreted
by their literal terms, their value would be greatly dimin-
ished. Unimportant and insubstantial substitutes for cer-
tain elements could defeat the patent, and its value to inven-
tors could be destroyed by simple acts of copying. For this

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reason, the clearest rule of patent interpretation, literalism,
may conserve judicial resources but is not necessarily the
most efficient rule. The scope of a patent is not limited to
its literal terms but instead embraces all equivalents to the
claims described. See Winans v. Denmead, 15 How. 330,
347 (1854).
It is true that the doctrine of equivalents renders the
scope of patents less certain. It may be difficult to deter-
mine what is, or is not, an equivalent to a particular element
of an invention. If competitors cannot be certain about a
patent’s extent, they may be deterred from engaging in legit-
imate manufactures outside its limits, or they may invest by
mistake in competing products that the patent secures. In
addition the uncertainty may lead to wasteful litigation be-
tween competitors, suits that a rule of literalism might avoid.
These concerns with the doctrine of equivalents, however,
are not new. Each time the Court has considered the doc-
trine, it has acknowledged this uncertainty as the price of
ensuring the appropriate incentives for innovation, and it has
affirmed the doctrine over dissents that urged a more certain
rule. When the Court in Winans v. Denmead, supra, first
adopted what has become the doctrine of equivalents, it
stated that “[t]he exclusive right to the thing patented is not
secured, if the public are at liberty to make substantial cop-
ies of it, varying its form or proportions.” Id., at 343. The
dissent argued that the Court had sacrificed the objective of
“[f]ul[l]ness, clearness, exactness, preciseness, and particu-
larity, in the description of the invention.” Id., at 347 (opin-
ion of Campbell, J.).
The debate continued in Graver Tank & Mfg. Co. v. Linde
Air Products Co., 339 U. S. 605 (1950), where the Court reaf-
firmed the doctrine. Graver Tank held that patent claims
must protect the inventor not only from those who produce
devices falling within the literal claims of the patent but also
from copyists who “make unimportant and insubstantial
changes and substitutions in the patent which, though adding

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nothing, would be enough to take the copied matter outside
the claim, and hence outside the reach of law.” Id., at 607.
Justice Black, in dissent, objected that under the doctrine of
equivalents a competitor “cannot rely on what the language
of a patent claims. He must be able, at the peril of heavy
infringement damages, to forecast how far a court relatively
unversed in a particular technological field will expand the
claim’s language . . . .” Id., at 617.
Most recently, in Warner-Jenkinson, the Court reaffirmed
that equivalents remain a firmly entrenched part of the set-
tled rights protected by the patent. A unanimous opinion
concluded that if the doctrine is to be discarded, it is Con-
gress and not the Court that should do so:
“[T]he lengthy history of the doctrine of equivalents
strongly supports adherence to our refusal in Graver
Tank to find that the Patent Act conflicts with that doc-
trine. Congress can legislate the doctrine of equiva-
lents out of existence any time it chooses. The various
policy arguments now made by both sides are thus best
addressed to Congress, not this Court.” 520 U. S., at
28.
III
Prosecution history estoppel requires that the claims of a
patent be interpreted in light of the proceedings in the PTO
during the application process. Estoppel is a “rule of patent
construction” that ensures that claims are interpreted by
reference to those “that have been cancelled or rejected.”
Schriber-Schroth Co. v. Cleveland Trust Co., 311 U. S. 211,
220–221 (1940). The doctrine of equivalents allows the pat-
entee to claim those insubstantial alterations that were not
captured in drafting the original patent claim but which
could be created through trivial changes. When, however,
the patentee originally claimed the subject matter alleged to
infringe but then narrowed the claim in response to a rejec-
tion, he may not argue that the surrendered territory com-

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prised unforeseen subject matter that should be deemed
equivalent to the literal claims of the issued patent. On the
contrary, “[b]y the amendment [the patentee] recognized and
emphasized the difference between the two phrases[,] . . . and
[t]he difference which [the patentee] thus disclaimed must be
regarded as material.” Exhibit Supply Co. v. Ace Patents
Corp., 315 U. S. 126, 136–137 (1942).
A rejection indicates that the patent examiner does not
believe the original claim could be patented. While the pat-
entee has the right to appeal, his decision to forgo an appeal
and submit an amended claim is taken as a concession that
the invention as patented does not reach as far as the origi-
nal claim. See Goodyear Dental Vulcanite Co. v. Davis, 102
U. S. 222, 228 (1880) (“In view of [the amendment] there can
be no doubt of what [the patentee] understood he had pat-
ented, and that both he and the commissioner regarded the
patent to be for a manufacture made exclusively of vulcanites
by the detailed process”); Wang Laboratories, Inc. v. Mitsu-
bishi Electronics America, Inc., 103 F. 3d 1571, 1577–1578
(CA Fed. 1997) (“Prosecution history estoppel . . . preclud[es]
a patentee from regaining, through litigation, coverage of
subject matter relinquished during prosecution of the appli-
cation for the patent”). Were it otherwise, the inventor
might avoid the PTO’s gatekeeping role and seek to recap-
ture in an infringement action the very subject matter sur-
rendered as a condition of receiving the patent.
Prosecution history estoppel ensures that the doctrine of
equivalents remains tied to its underlying purpose. Where
the original application once embraced the purported equiva-
lent but the patentee narrowed his claims to obtain the pat-
ent or to protect its validity, the patentee cannot assert that
he lacked the words to describe the subject matter in ques-
tion. The doctrine of equivalents is premised on language’s
inability to capture the essence of innovation, but a prior
application describing the precise element at issue undercuts
that premise. In that instance the prosecution history has

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established that the inventor turned his attention to the sub-
ject matter in question, knew the words for both the broader
and narrower claim, and affirmatively chose the latter.
A
The first question in this case concerns the kinds of amend-
ments that may give rise to estoppel. Petitioner argues
that estoppel should arise when amendments are intended
to narrow the subject matter of the patented invention, for
instance, amendments to avoid prior art, but not when the
amendments are made to comply with requirements con-
cerning the form of the patent application. In Warner-
Jenkinson we recognized that prosecution history estoppel
does not arise in every instance when a patent application
is amended. Our “prior cases have consistently applied
prosecution history estoppel only where claims have been
amended for a limited set of reasons,” such as “to avoid the
prior art, or otherwise to address a specific concern—such
as obviousness—that arguably would have rendered the
claimed subject matter unpatentable.” 520 U. S., at 30–32.
While we made clear that estoppel applies to amendments
made for a “substantial reason related to patentability,” id.,
at 33, we did not purport to define that term or to catalog
every reason that might raise an estoppel. Indeed, we
stated that even if the amendment’s purpose were unrelated
to patentability, the court might consider whether it was the
kind of reason that nonetheless might require resort to the
estoppel doctrine. Id., at 40–41.
Petitioner is correct that estoppel has been discussed most
often in the context of amendments made to avoid the prior
art. See Exhibi t Supply Co., supra, at 137; Keystone
Driller Co. v. Northwest Engineering Corp., 294 U. S. 42, 48
(1935). Amendment to accommodate prior art was the
emphasis, too, of our decision in Warner-Jenkinson, supra,
at 30. It does not follow, however, that amendments for
other purposes will not give rise to estoppel. Prosecution

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history may rebut the inference that a thing not described
was indescribable. That rationale does not cease simply
because the narrowing amendment, submitted to secure a
patent, was for some purpose other than avoiding prior art.
We agree with the Court of Appeals that a narrowing
amendment made to satisfy any requirement of the Patent
Act may give rise to an estoppel. As that court explained,
a number of statutory requirements must be satisfied before
a patent can issue. The claimed subject matter must be use-
ful, novel, and not obvious. 35 U. S. C. §§ 101–103 (1994 ed.
and Supp. V). In addition, the patent application must de-
scribe, enable, and set forth the best mode of carrying out
the invention. § 112 (1994 ed.). These latter requirements
must be satisfied before issuance of the patent, for exclusive
patent rights are given in exchange for disclosing the inven-
tion to the public. See Bonito Boats, 489 U. S., at 150–151.
What is claimed by the patent application must be the same
as what is disclosed in the specification; otherwise the patent
should not issue. The patent also should not issue if the
other requirements of § 112 are not satisfied, and an appli-
cant’s failure to meet these requirements could lead to the
issued patent being held invalid in later litigation.
Petitioner contends that amendments made to comply with
§ 112 concern the form of the application and not the subject
matter of the invention. The PTO might require the appli-
cant to clarify an ambiguous term, to improve the translation
of a foreign word, or to rewrite a dependent claim as an
independent one. In these cases, petitioner argues, the ap-
plicant has no intention of surrendering subject matter and
should not be estopped from challenging equivalent devices.
While this may be true in some cases, petitioner’s argument
conflates the patentee’s reason for making the amendment
with the impact the amendment has on the subject matter.
Estoppel arises when an amendment is made to secure the
patent and the amendment narrows the patent’s scope. If a
§ 112 amendment is truly cosmetic, then it would not narrow

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the patent’s scope or raise an estoppel. On the other hand,
if a § 112 amendment is necessary and narrows the patent’s
scope—even if only for the purpose of better description—
estoppel may apply. A patentee who narrows a claim as a
condition for obtaining a patent disavows his claim to the
broader subject matter, whether the amendment was made
to avoid the prior art or to comply with § 112. We must
regard the patentee as having conceded an inability to claim
the broader subject matter or at least as having abandoned
his right to appeal a rejection. In either case estoppel
may apply.
B
Petitioner concedes that the limitations at issue—the seal-
ing rings and the composition of the sleeve—were made for
reasons related to § 112, if not also to avoid the prior art.
Our conclusion that prosecution history estoppel arises when
a claim is narrowed to comply with § 112 gives rise to the
second question presented: Does the estoppel bar the inven-
tor from asserting infringement against any equivalent to
the narrowed element or might some equivalents still in-
fringe? The Court of Appeals held that prosecution history
estoppel is a complete bar, and so the narrowed element must
be limited to its strict literal terms. Based upon its experi-
ence the Court of Appeals decided that the flexible-bar rule
is unworkable because it leads to excessive uncertainty and
burdens legitimate innovation. For the reasons that follow,
we disagree with the decision to adopt the complete bar.
Though prosecution history estoppel can bar a patentee
from challenging a wide range of alleged equivalents made
or distributed by competitors, its reach requires an examina-
tion of the subject matter surrendered by the narrowing
amendment. The complete bar avoids this inquiry by estab-
lishing a per se rule; but that approach is inconsistent with
the purpose of applying the estoppel in the first place—to
hold the inventor to the representations made during the
application process and to the inferences that may reason-

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ably be drawn from the amendment. By amending the ap-
plication, the inventor is deemed to concede that the patent
does not extend as far as the original claim. It does not
follow, however, that the amended claim becomes so perfect
in its description that no one could devise an equivalent.
After amendment, as before, language remains an imperfect
fit for invention. The narrowing amendment may demon-
strate what the claim is not; but it may still fail to cap-
ture precisely what the claim is. There is no reason why
a narrowing amendment should be deemed to relinquish
equivalents unforeseeable at the time of the amendment and
beyond a fair interpretation of what was surrendered. Nor
is there any call to foreclose claims of equivalence for aspects
of the invention that have only a peripheral relation to the
reason the amendment was submitted. The amendment
does not show that the inventor suddenly had more foresight
in the drafting of claims than an inventor whose application
was granted without amendments having been submitted.
It shows only that he was familiar with the broader text and
with the difference between the two. As a result, there is
no more reason for holding the patentee to the literal terms
of an amended claim than there is for abolishing the doctrine
of equivalents altogether and holding every patentee to the
literal terms of the patent.
This view of prosecution history estoppel is consistent
with our precedents and respectful of the real practice before
the PTO. While this Court has not weighed the merits of
the complete bar against the flexible bar in its prior cases,
we have consistently applied the doctrine in a flexible way,
not a rigid one. We have considered what equivalents were
surrendered during the prosecution of the patent, rather
than imposing a complete bar that resorts to the very literal-
ism the equivalents rule is designed to overcome. E. g.,
Goodyear Dental Vulcanite Co., 102 U. S., at 230; Hurlbut v.
Schillinger, 130 U. S. 456, 465 (1889).

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The Court of Appeals ignored the guidance of Warner-
Jenkinson, which instructed that courts must be cautious
before adopting changes that disrupt the settled expecta-
tions of the inventing community. See 520 U. S., at 28. In
that case we made it clear that the doctrine of equivalents
and the rule of prosecution history estoppel are settled
law. The responsibility for changing them rests with Con-
gress. Ibid. Fundamental alterations in these rules risk
destroying the legitimate expectations of inventors in their
property. The petitioner in Warner-Jenkinson requested
another bright-line rule that would have provided more cer-
tainty in determining when estoppel applies but at the cost
of disrupting the expectations of countless existing patent
holders. We rejected that approach: “To change so substan-
tially the rules of the game now could very well subvert the
various balances the PTO sought to strike when issuing the
numerous patents which have not yet expired and which
would be affected by our decision.” Id., at 32, n. 6; see also
id., at 41 (Ginsburg, J., concurring) (“The new presumption,
if applied woodenly, might in some instances unfairly dis-
count the expectations of a patentee who had no notice at
the time of patent prosecution that such a presumption
would apply”). As Warner-Jenkinson recognized, patent
prosecution occurs in the light of our case law. Inventors
who amended their claims under the previous regime had no
reason to believe they were conceding all equivalents. If
they had known, they might have appealed the rejection in-
stead. There is no justification for applying a new and more
robust estoppel to those who relied on prior doctrine.
In Warner-Jenkinson we struck the appropriate balance
by placing the burden on the patentee to show that an
amendment was not for purposes of patentability:
“Where no explanation is established, however, the
court should presume that the patent application had a
substantial reason related to patentability for including
the limiting element added by amendment. In those

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circumstances, prosecution history estoppel would bar
the application of the doctrine of equivalents as to that
element.” Id., at 33.
When the patentee is unable to explain the reason for amend-
ment, estoppel not only applies but also “bar[s] the applica-
tion of the doctrine of equivalents as to that element.” Ibid.
These words do not mandate a complete bar; they are limited
to the circumstance where “no explanation is established.”
They do provide, however, that when the court is unable
to determine the purpose underlying a narrowing amend-
ment—and hence a rationale for limiting the estoppel to the
surrender of particular equivalents—the court should pre-
sume that the patentee surrendered all subject matter be-
tween the broader and the narrower language.
Just as Warner-Jenkinson held that the patentee bears
the burden of proving that an amendment was not made for
a reason that would give rise to estoppel, we hold here that
the patentee should bear the burden of showing that the
amendment does not surrender the particular equivalent in
question. This is the approach advocated by the United
States, see Brief for United States as Amicus Curiae 22–28,
and we regard it to be sound. The patentee, as the author
of the claim language, may be expected to draft claims en-
compassing readily known equivalents. A patentee’s deci-
sion to narrow his claims through amendment may be pre-
sumed to be a general disclaimer of the territory between
the original claim and the amended claim. Exhibit Supply,
315 U. S., at 136–137 (“By the amendment [the patentee]
recognized and emphasized the difference between the two
phrases and proclaimed his abandonment of all that is em-
braced in that difference”). There are some cases, however,
where the amendment cannot reasonably be viewed as sur-
rendering a particular equivalent. The equivalent may
have been unforeseeable at the time of the application; the
rationale underlying the amendment may bear no more than
a tangential relation to the equivalent in question; or there

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may be some other reason suggesting that the patentee could
not reasonably be expected to have described the insubstan-
tial substitute in question. In those cases the patentee can
overcome the presumption that prosecution history estoppel
bars a finding of equivalence.
This presumption is not, then, just the complete bar by
another name. Rather, it reflects the fact that the interpre-
tation of the patent must begin with its literal claims, and
the prosecution history is relevant to construing those
claims. When the patentee has chosen to narrow a claim,
courts may presume the amended text was composed with
awareness of this rule and that the territory surrendered
is not an equivalent of the territory claimed. In those
instances, however, the patentee still might rebut the
presumption that estoppel bars a claim of equivalence. The
patentee must show that at the time of the amendment one
skilled in the art could not reasonably be expected to have
drafted a claim that would have literally encompassed the
alleged equivalent.
IV
On the record before us, we cannot say petitioner has re-
butted the presumptions that estoppel applies and that the
equivalents at issue have been surrendered. Petitioner con-
cedes that the limitations at issue—the sealing rings and the
composition of the sleeve—were made in response to a rejec-
tion for reasons under § 112, if not also because of the prior
art references. As the amendments were made for a reason
relating to patentability, the question is not whether estoppel
applies but what territory the amendments surrendered.
While estoppel does not effect a complete bar, the question
remains whether petitioner can demonstrate that the nar-
rowing amendments did not surrender the particular equiva-
lents at issue. On these questions, SMC may well prevail,
for the sealing rings and the composition of the sleeve both
were noted expressly in the prosecution history. These
matters, however, should be determined in the first instance

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by further proceedings in the Court of Appeals or the Dis-
trict Court.
The judgment of the Federal Circuit is vacated, and the
case is remanded for further proceedings consistent with
this opinion.
It is so ordered.