Essentially, means plus function claiming allows the drafter to claim the invention based on functionality rather than the more traditional (and preferred) claiming technique that employs structure within the body of the claim itself.

A claim term is functional when it recites a feature by what it does rather than by what it is. There is nothing intrinsically wrong with the use of such claim language, although it is becoming more and more apparent (if it wasn’t already apparent enough) that the Federal Circuit frowns upon means plus function claiming.

Nevertheless, 35 U.S.C. § 112(f)[1] expressly authorizes a form of functional claiming (means plus function claiming). The statute says:

An element in a claim for a combination may be expressed as a means or step for performing a specified function without the recital of structure, material, or acts in support thereof, and such claim shall be construed to cover the corresponding structure, material, or acts described in the specification and equivalents thereof.

The basic law relative to § 112(f) explains that a decision on whether a claim is indefinite under § 112(f) requires a determination of whether those skilled in the art would understand what is claimed when the claim is read in light of the specification. Traditionally, claim terms are typically given their ordinary and customary meaning as understood by one of ordinary skill in the pertinent art. The question with means plus function claiming, however, is whether evidence from that mythical individual skilled in the art is even admissible.

If there is no structure in the specification the person of skill in the art cannot save the disclosure by understanding what the drafter intended to be covered by the means plus function limitation in the claims. Thus, means-plus-function claims are valid at the mercy of the specification, and only to the extent that the specification contains support for the structures that define the means. The broadest reasonable interpretation of a means plus function claim will, therefore, be drawn to the structure, material or acts described in the specification and their equivalents. MPEP 2181.

So how do you know when you will get hit with the peculiar disclosure requirements invoked when you rely upon § 112(f)? If a claim limitation recites a term and associated functional language, the examiner should determine whether the claim limitation invokes § 112(f). The claim limitation is presumed to invoke § 112(f) when it explicitly uses the phrase “means for” or “step for” and includes functional language. That presumption is overcome when the limitation further includes the structure necessary to perform the recited function.

By contrast, a claim limitation that does not use the phrase “means for” or “step for” will trigger a rebuttable presumption that § 112(f) does not apply. This presumption is a strong one that is not readily overcome, and, as such, the Federal Circuit rarely has found a claim limitation to trigger means plus function treatment without the recitation of “means” language in the claim itself. Apple Inc. v. Motorola, Inc., 757 F.3d 1286 (Fed. Cir. 2014).

Furthermore, means treatment will not apply if persons of ordinary skill in the art reading the specification understands the term used to be the name for the structure that performs the function, “even if the term covers a broad class of structures and even if the term identifies structures by their function.” TecSec, Inc. v. Int’l Bus. Machs. Corp., 731 F.3d 1336, 1347 (Fed. Cir. 2013). In this regard the term is not required to denote any specific structure or a precise physical structure in order to avoid means plus function treatment. In TecSec the Federal Circuit concluded that “system memory means” and “digital logic means” did not invoke means plus function treatment, specifically explaining that “a system memory is a specific structure that stores data,” and that digital logic means was specifically disclosed in the specification to be “comprised of structural elements, including a system memory and specific modules and subsystems.”

Similarly, in Skky, Inc. v. Mindgeek, S.A.R.L., 859 F.3d 1014 (Fed. Cir. 2017), the Federal Circuit determined that “wireless device means” did not invoke means treatment because sufficient structure was recited. Although the term in question specifically used the word “means” and, therefore, triggered the initial presumption that means treatment was appropriate, the full term was found to recite structure, not to be merely functional. The Federal Circuit, citing TecSec, explained that the term “wireless device” is used in common practice to designate structure, which made it inappropriate to apply means plus function treatment to the claim.

Of course, as implied by the previous two cases, which used the word “means” but were found not to invoke means plus function treatment, the Federal Circuit does not blindly elevate form over substance when evaluating whether a particular claim limitation invokes means treatment. See Greenberg v. Ethicon Endo-Surgery, Inc., 91 F.3d 1580, 1584 (Fed. Cir. 1996) (“We do not mean to suggest that section 112(6) is triggered only if the claim uses the word ‘means.’”).

As the cases repeatedly explain, determining whether a claim limitation properly invokes means plus function treatment cannot merely focus on the presence or absence of the word “means.” Focus must be appropriately placed on “whether the words of the claim are understood by persons of ordinary skill in the art to have a sufficiently definite meaning as the name for structure.” Williamson v. Citrix Online, LLC, 792 F.3d 1339 (Fed. Cir. 2015). This was, in fact, why the Federal Circuit ruled in Skky “wireless device means” did not invoke means treatment.

Therefore, what is important is whether the term used is one that is understood to describe structure, although it might not bring to mind any particular structure. For example, the court in Personalized Media Communications v. International Trade Commission, 161 F.3d 696 (Fed. Cir. 1999), held that the term “detector,” although broad, is still structural for purposes of means treatment because it is not a generic structural term such as “means,” nor is it a coined term lacking a clear meaning. Similarly, in Lighting World, Inc. v. Birchwood Lighting, Inc., 382 F.3d 1354 (Fed. Cir. 2004), the Federal Circuit held the term “connector assembly” meant a unit that joins, fastens, or links each pair of adjacent support members. The fact that more than one structure may be described by the term “connector assembly,” or even that the term may encompass a multitude of structures, did not make it any less structural and somehow transform it into an attempt to functionally claim.

Given all of this, when a claim limitation does not use the phrase “means for” or “step for,” it is necessary to determine whether the claim limitation uses a nonstructural term (i.e., a term that is simply a substitute for the term “means for”). Examiners will apply § 112(f) to a claim limitation that uses a nonstructural term associated with functional language, unless the nonstructural term is (1) preceded by a structural modifier, defined in the specification as a particular structure or known by one skilled in the art, that denotes the type of structural device, or (2) modified by sufficient structure or material for achieving the claimed function.

The following is a list of non-structural terms that may invoke means plus function treatment: “mechanism for,” “module for,” “device for,” “unit for,” “component for,” “element for,” “member for,” “apparatus for,” “machine for,” or “system for.” This list is not exhaustive, and other non-structural terms may invoke § 112(f).

The following are examples of structural terms that have been found not to invoke means-plus-function treatment: “circuit for,” “detent mechanism,” “digital detector for,” “reciprocating member,” “connector assembly,” “perforation,” “sealingly connected joints,” and “eyeglass hanger member.”

A limitation will not invoke means plus function treatment if there is a structural modifier that further describes the non-structural term. For example, although a nonstructural term like “mechanism” standing alone may invoke means plus function treatment, when it is coupled with a function it will not invoke such treatment when it is preceded by a structural modifier (e.g., “detent mechanism”). By contrast, when a non-structural term is preceded by a non-structural modifier that does not have any generally understood structural meaning in the art, the phrase may invoke means treatment when coupled with a function (e.g., “colorant selection mechanism,” “lever moving element,” or “movable link member”).

To determine whether a word, term, or phrase coupled with a function denotes structure, it is necessary to check whether: (1) The specification provides a description sufficient to inform one of ordinary skill in the art that the term denotes structure; (2) general and subject matter specific dictionaries provide evidence that the term has achieved recognition as a noun denoting structure; and (3) the prior art provides evidence that the term has an art-recognized structure to perform the claimed function.

Examiners will apply means treatment to a claim limitation that meets the following conditions: (1) The claim limitation uses the phrase “means for” or “step for” or a non-structural term that does not have a structural modifier; (2) the phrase “means for” or “step for” or the non-structural term recited in the claim is modified by functional language; and (3) the phrase “means for” or “step for” or the non-structural term recited in the claim is not modified by sufficient structure, material, or acts for achieving the specified function.

When it is unclear whether a claim limitation invokes means treatment or not, a rejection under § 112(b) may be appropriate. Similarly, when applicant uses the phrase “means for” or “step for” in the preamble, a rejection under § 112(b) may be appropriate when it is unclear whether the preamble is reciting a means (or step) plus function limitation or whether the preamble is merely stating the intended use of the claimed invention. Of course, when an applicant uses a structural or nonstructural term with the word “for” in the preamble, the examiner should not construe such phrase as reciting a means plus function limitation.

Once the examiner determines that a claim limitation is a means plus function limitation invoking § 112(f), the examiner should determine the claimed function and then review the written description of the specification to determine whether the corresponding structure, material, or acts that perform the claimed function are disclosed. The disclosure must be reviewed from the point of view of one skilled in the relevant art to determine whether that person would understand the written description to disclose the corresponding structure, material, or acts. To satisfy the definiteness requirement under § 112(f), the written description must clearly link or associate the corresponding structure, material, or acts to the claimed function. A rejection under § 112(b) is appropriate if the written description fails to link or associate the disclosed structure, material, or acts to the claimed function, or if there is no disclosure (or insufficient disclosure) of structure, material, or acts for performing the claimed function. A bare statement that known techniques or methods can be used would not be a sufficient disclosure to support a means plus function limitation.

A rejection under § 112(b) may be appropriate in the following situations when examining means plus function claim limitations under § 112(f): (1) When it is unclear whether a claim limitation invokes § 112(f); (2) when § 112(f) is invoked and there is no disclosure or there is insufficient disclosure of structure, material, or acts for performing the claimed function; and/or (3) when § 112(f) is invoked and the supporting disclosure fails to clearly link or associate the disclosed structure, material, or acts to the claimed function.

When the examiner cannot identify the corresponding structure, material, or acts, a rejection under § 112(b) should be made, thus rendering the means plus function claim indefinite. If the written description sets forth the corresponding structure, material, or acts in compliance with § 112(b), the claim limitation must be construed to cover the corresponding structure, material, or acts described in the specification and equivalents thereof.

So what should you do?

Time and time again in patent practice you are always better off filing the most complete disclosure you possibly can at the earliest date possible. Describe things generally and build to ever more specific embodiments and alternative descriptions, explaining all the variations and possibilities with as much detail as you possibly can. This is the winning approach even if you do not use means plus function claims, but it is absolutely essential if you are going to use means plus function claims. Of course, remember that high quality patent drawings that show structures can be a highly effective way to avoid indefiniteness rejections.

Should you avoid means plus function claims? While many will advice yes, the best answer is more nuanced.

Means plus function claiming is an excellent way to make sure that you have captured within the claims all of the various means disclosed in the application. Claiming all the possible permutations can be extremely costly, so using a couple of means-plus-function claims in an attempt to have the claim scope as broad as the disclosure can be a very good strategy. Means plus function claims cannot, however, be the primary claims and for goodness sakes don’t file patent applications with only means-plus-function claims. Means-plus-function should be used like seasoning — a very powerful garlic! A little may be OK but too much is a very poor choice.

Of course, it is critical to also understand that the rules for means plus function claiming apply very differently with respect to software and computer implemented inventions. See The Algorithm Cases.

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[1] This article cites to 35 U.S.C. 112(f), which today is the proper citation under the Leahy-Smith America Invents Act. The article also generally refers to “means plus function treatment” and “means treatment” to apply to both 35 U.S.C. 112(f) and 35 U.S.C. 112, ¶ 6. Of course, prior to enactment of the AIA the proper citation was 35 U.S.C. 112, ¶ 6. After, the AIA the proper citation became 35 U.S.C. 112(f).