The Strategic Use Of Means-Plus-Function Claims

Apr 1998The Journal of the Patent and Trademark Office Society, volume 80, April 1998, pages 241-250


by Gregory J. Maier and Bradley D. Lytle*


Claim drafting is perhaps the most complex and sophisticated form of legal writing. There are several reasons for this. One reason is that claims often deal with very sophisticated technology, and so drafting proper claims requires an in-depth understanding of complex technology. Another reason is that claims do not simply describe technology in a direct or literal sense, but seek to broadly define what is new in the technology while distinguishing what is conventional. Thus, claim drafting requires not only understanding the essence of current technology, but also understanding the essence of past technology, then drawing in words the perceived distinction between current and past technology.

Moreover, creating valuable intellectual property requires more than skill in drafting individual claims; it requires an overall strategy that attempts to anticipate the future. To understand the need of a forward-looking claim drafting strategy, it is helpful to contrast the processes of claim drafting and patent enforcement.

When drafting patent claims, ideally, the drafting attorney is aware of the state of the art as well as the essence of the new technology to be patented. The drafter is also aware of available prior art and of the current state of the law. Taking into account all of these known parameters, the drafter is poised to prepare suitable claims. The value of those claims, however, is affected by whether the drafter has taken into account the fact that patent claims are often enforced as much as a decade after their drafting‹and sometimes several decades after the claims were drafted.[1] During the decade or decades between claim drafting and patent enforcement, many court opinions will have issued, most probably changing to a degree the U.S. law on claim interpretation. As a further complication, the technology itself usually evolves‹often spectacularly.

Another aspect of the patent enforcement process is the presumption that the claimed invention has developed a commercial value that justifies the substantial cost of litigation or extensive licensing efforts. One of the practical implications of this is that, generally, new prior art is located after thorough searching or through the litigation discovery process. Thus, generally speaking, the prior art available when the patent is enforced is far more extensive (and more relevant) than the prior art that was available when the claims were originally drafted. Consequently, available prior art, the technology itself, and the law generally have all changed dramatically between claim drafting and patent enforcement.

The challenge facing the claim drafter is therefore not only to write the best claims possible given the initial state of the prior art, the technology, and the law known when drafting the claims, but to select a drafting strategy that will give the patent owner the best chance of having strong, enforceable rights in the future, when the technology, prior art, and patent law have all evolved to a state that could not have been predicted when the claims were originally drafted.


In a nutshell, we believe that the best claim drafting strategy to achieve this end involves using all available claiming techniques in order to best take advantage of the different law applicable under each claiming technique. Presenting apparatus claims written in plain language, apparatus claims written in means-plus-function language, method claims and computer program product claims (i.e., all types of claims relevant to the subject technology) is what we recommend.

However, this paper is not intended to deal with every aspect of claiming strategy. Rather, we focus on the use of means-plus-function claims (i.e., "Means claims"[2]) as one component of a claiming strategy. We have taken this focus because there has been considerable recent case law interpreting Means claims that, while clarifying some points, has actually created significant confusion among some practitioners as to how Means claims may be effectively used as a component of a claim drafting strategy. This confusion is perhaps accentuated by some commentators who implore practitioners to "use extreme caution" when drafting claims,[3] which, in some cases, has discouraged practitioners from using Means claims altogether. We strongly disagree with the philosophy that use of Means claims should be discouraged and maintain that the use of Means claims is an important aspect of a truly sophisticated and forward-looking claiming strategy.

In the wake of In re Donaldson[4] and related cases,[5] we believe that many patent practitioners have been left with the impression that Means claims are "too narrow" to have any commercial significance. We disagree with this view, and we explain how, even under present law (let alone the law that may be prevalent when claims drafted today are being litigated), Means claims can be used to offer expanded claim scope, bolster patent validity, and preserve infringement arguments that might not otherwise be available. For the reasons explained below, the claim drafting strategy we recommend includes supplementing sets of plain language claims (e.g., method, non-Means apparatus and non-Means computer program product claims) with related Means claims as a way to more completely secure intellectual property rights for the owner of an invention.


Although recent case law has focused attention on the scope of Means claims as being sometimes narrower than a literal interpretation of the language used in the Means claims, the scope of a Means claim is not a fixed quantity, but can vary considerably depending upon the disclosure to which it refers. Three illustrative examples follow.

The first example is that of a straight-forward mechanical invention, where the disclosed embodiment is quite specific and alternative structures are absent. In this example, plain language claims will typically be broader than Means claims.[6] This is the situation that several post-In re Donaldson[7] cases have dealt with, and this is the situation that has given Means claims a negative reputation. In this situation Means claims are construed as including the structure actually disclosed, which in the straight-forward mechanical example is fairly specific. Of course, plain language claims are not so construed. Thus, for the straight forward mechanical invention, the detailed structural embodiment disclosed may have a narrowing effect on the scope of a Means claim. This effect may be pronounced, especially if the specification fails to describe in broad terms the function accomplished by the disclosed structure and fails to identify other approaches for performing the same function.

As a second example, suppose the disclosed invention is a system, or a software invention having interrelated functional components (best illustrated by a block diagram), where the actual structure shown is of ancillary importance to the functional interrelationship of the components. Under these conditions, there may be no practical difference in scope between Means claims and plain language claims.[8] The different types of claims would most probably be construed to have similar scope because the specification is primarily written in broad functional terms, which are reflected in the Means claims. In this example there is no detailed structural disclosure that will be "read into" the Means claim to narrow its scope significantly.

As a third example, consider the following situation. Assume that the disclosed invention requires a source of digital information and that the specification states that the digital information may be supplied by either certain enumerated types of information storage devices (e.g., local sources such as a magnetic disk drive, semiconductor memory, etc.) or a transmission line carrying live data from a remote source. It would be difficult to draft plain language claims that are broad enough to cover all disclosed alternatives while succinct enough to achieve the desired scope. To recite "a source of digital information" would be too broad in this example because not all possible sources or even all types of storage devices are contemplated by the invention. Reciting "a selected information storage device or a transmission line" is a possible approach, but it is awkward and could result in a §112 rejection during prosecution as indefinite for use of alternative language. Accordingly, obtaining the desired scope with a plain language claim may be challenging due to the inherent restrictions associated with using "labels" to define a claimed feature. On the other hand, claiming "means for supplying digital information" covers the disclosed invention simply and completely. This example illustrates how the Means claim format may be used to overcome limitations in the use of plain language, and to conveniently achieve the desired scope of protection for a complex invention.[9]

Consequently, it is too simplistic to conclude that Means claims are always of narrower scope than plain language claims. The truth is that the scope of Means claims is heavily dependent on the nature of the invention, how the invention is characterized in the disclosure, and the degree to which alternatives are disclosed in the specification. Thus we believe that, even under current law, Means claims cannot be dismissed as being "too narrow" in scope to be useful. Furthermore, comparing only the relative breadth of Means and plain language claims is itself a simplistic approach to the broader question as to whether Means claims have a place in claim drafting strategy. Finally, we think it possible (even probable) that by the time that a claim drafted today is litigated, the courts' interpretation of Means claims may have changed dramatically yet again.


In litigation or licensing negotiations, the worst case for the patent holder is to have all of the claims invalidated[10] because then the patent becomes totally useless, ending all negotiations and all hope of receiving royalty payments. On the other hand, if even only one claim survives a validity attack, it poses a potential threat to the accused infringer because, even though the claim may be of limited scope, the doctrine of equivalents and the possibility of litigation before a jury are "wild cards" that create a risk factor the accused infringer cannot ignore. Thus, a sophisticated claiming strategy should not be limited only to obtaining the broadest possible claims, but should also seek to include one or more claims that are very difficult to invalidate. Again, Means claims may prove to be valuable in this role because they can be the most difficult claims to invalidate. Since the scope of these claims may depend on the details disclosed in the specification as pointed out above, prior art that is virtually identical to the disclosed device may be required to overcome the presumption of validity and render the Means claims unquestionably invalid. Consequently, the inclusion of Means claims as a strategic defense to invalidity is prudent and advisable.


Inventions are often best described in terms of several different embodiments, and often each embodiment may itself include several alternatives.[11] The claim drafter is challenged to originate claims that are sufficiently broad in scope to cover all novel features of each aspect and embodiment of the disclosed invention, while at the same time distinguishing the prior art.

Including only plain language claims gives rise to the risk of not including one or more of the embodiments or alternative components disclosed in the specification.[12] Further, even if the originally drafted plain language claims fully cover all of the embodiments and alternative components, inadvertently excluding one or more of the embodiments during the prosecution remains a risk factor.[13] In many cases, such claiming omissions may go unnoticed until the patent is enforced‹leading to very unfortunate consequences![14]

Including Means claims provides protection against failing to claim any disclosed embodiment or alternative structure because Means claims are legally construed to cover the structure, material, or acts actually described in the specification. Means claims need only describe the function being performed, yet all disclosed embodiments that perform the claimed function are covered by the claims.[15] The Means claim is therefore valuable as a kind of insurance, or risk mitigation vehicle, against failure to cover all disclosed alternatives and embodiments.


There are a number of legal principles and trends that further suggest the wisdom of including some claims in Means format as a part of a sophisticated claiming strategy. One principle is the doctrine of claim differentiation, under which U. S. courts strive to find different meaning and scope in claims having different, but similar, wording.[16] One possible result may be that including a Means claim persuades a court to assign a broader interpretation to a similarly worded claim written in plain language. Thus, the Means claim may indirectly broaden the scope of the overall claim package. Alternatively, if a plain language claim is found invalid on the basis of newly discovered prior art, under the doctrine of claim differentiation, a court may assign a narrower scope to a similarly worded Means claim, thereby saving the Means claim from an invalidity holding.

Another evolving area of law centers on the definition of the term "equivalents" as it is used in 35 U.S.C. § 112, ¶ 6. Presently there is no precise definition of this term, although Federal Circuit precedent is that § 112 equivalents should be interpreted differently from equivalents under the "doctrine of equivalents".[17] While it is impossible to anticipate exactly how the law on this topic will evolve and how it might affect individual patent claims, it is prudent to include Means claims in an application so as to preserve the possibility of calling on any favorable law that may develop.

Another developing area of interest is the question of jury access. Presently, the U.S. Supreme Court has determined that interpretation of claims is reserved for the judge,[18] while the question of determining infringement via equivalency under the doctrine of equivalents remains a fact issue for the jury.[19] However, no post-Markman case has specifically addressed the question of whether a jury will be entitled to decide the issue of infringement under § 112 equivalents.[20] Although this issue could be reserved for determination by the judge, the possibility remains that this could be a jury issue. Since the patentee's position is often most favorably considered by a jury, it is wise to preserve this option by including Means claims.


The important In re Donaldson[21] decision and its many progeny[22] have focused extensive attention on the interpretation of Means claims in the last few years. Generally, these cases contain a detailed analysis of only one or two selected claims. Moreover, these cases have often focused on the narrow scope of some Means claim limitations. The general impression imparted on practitioners (particularly those outside the United States) by these cases and related commentary has been that Means claims are something to be feared, akin to a Trojan horse. Because the scope of Means claims is controlled by the specification, and is thus different from the literal language of the claims, some practitioners have jumped to the conclusion that Means claims are impossible to understand and should never be used. We feel that this conclusion is a serious mistake if it leads to entirely avoiding Means claims. Means claims, properly understood and prudently used, can add important richness and depth to a claim drafting strategy.

Concerning In re Donaldson[23] and its progeny,[24] it is our view that the case law is not always useful in providing guidance on claim drafting strategy. There are several reasons for this, the most important being that case law is by nature retrospective. It informs us only long after claims are planned and drafted whether or not a particular claim is valid or infringed, and it sheds little light on the efficacy of the planning strategy itself. Another reason is that the Court decision is rendered against a background of rigid fact‹where prior art, technology, and legal principles are all clearly known. It would be no challenge to write valid and infringed claims under those static and precisely defined circumstances! However, as pointed out above, these circumstances are essentially the opposite of those actually faced by the claim drafter. In reality, the drafter is faced with planning a claiming strategy that ideally projects the strongest possible rights package into a fluid and unpredictable future.

For the reasons advanced above, we believe that a sophisticated and forward-looking claiming strategy should include both Means claims and plain language claims. The extent to which either type of claim is used may vary considerably based on the type of invention sought to be protected, the specific disclosure, and the description in the disclosure of specific alternative structures. However, the inclusion of both Means claims and plain language claims will, in our view, provide a more reliable and effective scope of protection for an uncertain future than either type of claim used alone.

* Oblon, Spivak, McClelland, Maier & Neustadt, P.C. © 1997. Mr. Maier is a partner and Mr. Lytle is an associate in the firm.

Published in The Journal of the Patent and Trademark Office Society, volume 80, April 1998, pages 241-250.


[1].Aside from the time required to prosecute the application (on average 21 months, see USPTO 1996 Annual Review), time is usually required for the patentee to learn of the infringing activity and to assert the claims in litigation or licensing negotiations. Moreover, sometimes infringement does not begin until many years after the patent issues. For example, in the well known case of Texas Instruments, Inc. v. United States Int'l Trade Comm'n, 805 F.2d 1558, 231 U.S.P.Q. 833 (Fed. Cir. 1986), 17 years transpired between the filing of the patent specification (first filed in a parent application) and the filing of the complaint with the International Trade Commission.

[2].We refer to "Means claims" in a general sense as being claims and claim elements that invoke an interpretation under 35 U.S.C. § 112, ¶ 6. We capitalize the term "Means" only for emphasis and only when we are referring to the class of claims known as means-plus-function claims.

[3].See, e.g., Kuester, Jeffrey, R., et al., The New Means-Plus-Function Presumption and Step-Plus-Function Confusion, 1997 AIPLA Annual Meeting Papers. Also see, Yamuguchi, Yoichiro, and Wegner, Harold, C., Construing the "Means-Plus-Function" Claim in Current U.S. Legal Trends, Japanese AIPPI, Vol. 42, No. 1 (Japanese Language). See also, Smith, John G., Lecture on Treatment of Means-Plus-Function Claim Terminology under U.S. Patent Law for U.S.P.Q. Benkyo Kai, February 26, 1997, which describes pitfalls, as well as attributes, of Means claims, observing at page 7 that "many practitioners are now advising that drafting claims in this format should be avoided." In light of such analysis and commentary, many practitioners are uncertain as to the benefits offered by Means claims and whether Means claims should be used at all.

[4].16 F.3d 1189 (Fed. Cir. 1994) (en banc).

[5].For example, Greenberg v. Ethicon Endo-Surgery, Inc., 91 F.3d 1580 (Fed. Cir. 1996); York Products, Inc. v. Central Tractor Farm & Family Ctr. and Custom Form Mfg., 99 F.3d 1568 (Fed. Cir. 1996); Cole v. Kimberly-Clark Corp., 102 F.3d 524 (Fed. Cir. 1996); O.I. Corp. v. Tekmar Co. Inc., 115 F.3d 1576 (Fed. Cir. 1997); and Sage Products, Inc. v. Devon Industries, Inc., 126 F.3d 1420 (Fed. Cir. 1997).

[6].With respect to plain language claims, particular embodiments appearing in the specification will not be read into the claims when the claim language is broader than such embodiments. Electro Med. Sys. S.A. v. Cooper Life Sciences, Inc., 34 F.3d 1048 (Fed. Cir. 1994). Thus, plain language claims may be broader in scope than what is disclosed in the specification, provided the language actually used in the plain language claim permits as much.

[7].16 F.3d 1189 (Fed. Cir. 1994) (en banc).

[8].The point here is not that software-related inventions can only be described in functional terms, but rather to identify an example situation where the drafter opted to describe the invention primarily in terms of functional language. Regarding the interrelationship of computer related inventions, the U.S.P.T.O. recognizes that a claimed computer-readable medium encoded with a computer program defines a structural and functional interrelationship between the computer program and the medium. This interrelationship permits the computer program's functionality to be realized, and thus be patentable. M.P.E.P. § 2106 IV.B.1.(a). However, to the extent alternative components may be used to perform the described function, it would be wise to enumerate in the specification these alternate components and to explain that they may be used as well.

[9].We should also mention a technical difference between Means and plain language claims that may have an impact in measuring the respective range of equivalents associated with each type of claim. Plain language claim limitations are entitled to include a range of equivalents under the doctrine of equivalents, applied when the claims are not literally infringed. Means claims are entitled to the same range of equivalents under the doctrine of equivalents, and they are also statutorily entitled, in a literal infringement context, to be construed to cover "corresponding structure . . . described in the specification and equivalents thereof." Johnston v. IVAC Corp., 885 F.2d 1574 (Fed. Cir. 1997). The doctrine of equivalents is an equitable doctrine intended to prevent patent claims from being subverted by a mere colorable modification of an accused device. Under the doctrine of equivalents, the range of equivalents may be limited by prosecution history estoppel, a traditional estoppel theory requiring reliance (by the patent examiner) and action based on the reliance (allowance of a claim). See, e.g., Warner-Jenkinson Co. v. Hilton Davis Chem. Co., 117 S. Ct. 1040 (1997). In contrast, the statutory "equivalents" to which Means claims are entitled is created by law. 35 U.S.C. § 112, ¶ 6. A possible argument is that prosecution history estoppel does not apply to legal "equivalents" included in the scope of Means claims.

[10].A patent's claims define the legal rights a patent holder has in the invention. If all of the claims are invalidated, or held unenforceable, the patent becomes worthless because no claim remains to be asserted against accused infringers.

[11].This is particularly important when using Means claims because the scope of the Means claims will be determined by what is actually disclosed in the specification and equivalents to what is actually disclosed in the specification. Warner-Jenkinson Co. v. Hilton Davis Chem. Co., 117 S. Ct. 1040 (1997). Furthermore, when a judge determines the meaning of a claim, the judge will look first to the intrinsic evidence (i.e., the claims themselves, the written description portion of the specification, and the prosecution history), and will only look to extrinsic evidence in rare instances. See Vitronics Corp. v. Conceptronic, Inc., 90 F.3d 1576, 1581-82 (Fed. Cir. 1996), and Bell & Howell Document Mgmt. Prod. Co. v. Keystone Jackets, Inc., 1997 U.S. App. LEXIS 34253 (Fed. Cir. Dec. 5, 1997). Thus, the present case law encourages practitioners to file applications that completely describe the preferred embodiments as well as alternatives in ways that would be clear to a judge, without further explanation by way of expert testimony.

[12].The magnitude and effect of this risk can be quite sizeable, as evidenced, e.g., by Maxwell v. J. Baker, Inc., 86 F.3d 1098 (Fed. Cir. 1996). In that case, the United States Court of Appeals for the Federal Circuit (CAFC) determined that subject matter that is disclosed, but not adequately covered in the patent claims, may not later be recaptured by expanding the effective scope of the narrow claims under the doctrine of equivalents. Maxwell, 86 F.3d at 1108.

[13]. A first risk occurs when the attorney, while thinking the original claims cover all of the disclosed embodiments, inadvertently fails to disclose one or more of the embodiments when drafting the application. A second risk occurs when the attorney is aware of the invention to be claimed, but unintentionally claims the invention too narrowly or inaccurately. In this situation, it is particularly unlikely that anyone will discover the error in claim scope until the patent is asserted against another party, which may well be after expiration of the two-year period for seeking a broadening reissue application. 35 U.S.C. § 251. A third risk occurs when the attorney amends the claim during prosecution, but fails to recognize that the claims as amended exclude one or more embodiments of the invention.

[14]. Namely, a realization that the patent's claims do not read-on the accused device and a realization of a lost opportunity to obtain royalties for the invention or to prevent competitors from practicing the invention.

[15]. 35 U.S.C. § 112, ¶ 6 and Sage Products, Inc. v. Devon Industries, Inc., 126 F.3d 1420 (Fed. Cir. 1997).

[16]. See, e.g., Laitram Corp. v. Rexnord, Inc., 939 F.3d 1533 (CAFC 1991).

[17]. In re Donaldson Co., Inc., 16 F.3d 1189, 1195 n.8 (Fed. Cir. 1994); and Valmont Industries, Inc. v. Reinke Mfg. Co., 983 F.2d 1039, 1043 (Fed. Cir. 1993). However, the Supreme Court's analysis of "equivalents" under 35 U.S.C. § 112, ¶ 6 being "an application of the doctrine of equivalents in a restricted role" casts some doubt about how the Federal Circuit had previously distinguished "equivalents" under 35 U.S.C. § 112, ¶ 6 from the doctrine of equivalents. Warner-Jenkinson Co. v. Hilton Davis Chem. Co., 117 S. Ct. 1040, 1048 (1997).

[18]. Markman v. Westview Instruments, Inc., 116 S. Ct. 1384 (1996).

[19]. Warner-Jenkinson Co. v. Hilton Davis Chem. Co., 117 S. Ct. 1040, 1053 (1997).

[20]. Rephrasing the issue, is it the judge who decides the meaning of an "equivalent" under § 112, or does the jury interpret the meaning of an "equivalent" under § 112, ¶ 6 as part of its role in determining whether a Means claim is infringed? The Supreme Court declined to decide a similar issue with respect to the doctrine of equivalents in Warner-Jenkinson Co., id., noting that nothing in Markman necessitates a different result than the CAFC's precedent to charge the jury with deciding whether an accused device is equivalent to a claimed invention.

[21]. 16 F.3d 1189 (Fed. Cir. 1994) (en banc).

[22]. See supra note 5.

[23]. 16 F.3d 1189 (Fed. Cir. 1994) (en banc).

[24]. See supra note 5.