Means-plus-function

A limitation is presumed to invoke 35 U.S.C. 112(f) when it explicitly uses the term "means" or "step" and includes functional language. The presumption is overcome when the limitation also includes the structure, material or acts needed to perform the function². A limitation that does not use "means" or "step" triggers a rebuttable presumption that 112(f) does not apply².

In Williamson v. Citrix Online (Fed. Cir. 2015), the court, in a part decided en banc, expressly overruled the characterization of that second presumption as "strong" and said it would apply it without requiring any heightened evidentiary showing³. The presumption can be overcome and 112(f) will apply if the challenger shows that the claim term fails to recite sufficiently definite structure, or recites function without sufficient structure for performing it. The standard is 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 structure³.

The USPTO applies 112(f) when all three prongs of this analysis are met²:

1. A placeholder for "means"The limitation uses "means" or "step", or a generic placeholder (a nonce term with no specific structural meaning) for performing the claimed function².
2. Modified by functional languageThe term or placeholder is modified by functional language, typically linked by "for" or another linking word or phrase such as "configured to" or "so that"².
3. Not modified by sufficient structureThe term or placeholder is not modified by sufficient structure, material or acts for performing the claimed function².

The MPEP lists "mechanism for", "module for", "device for", "unit for", "element for", "apparatus for" and "system for" among non-structural generic placeholders that may invoke 112(f), and says there is no fixed list of words that always do or never do. A term that persons of ordinary skill understand as the name of a structure, such as "filters", "brakes", "clamp" or "locks", does not invoke it, even when it covers a broad class².

• The corresponding structure, material or acts must be disclosed in the specification itself, in a way that one skilled in the art will understand what structure will perform the recited function. If nothing is disclosed, the claim fails the definiteness requirement of 35 U.S.C. 112(b)².
• A bare statement that known techniques or methods can be used does not disclose structure².
• For a computer-implemented limitation, the specification must disclose an algorithm for performing the claimed specific computer function, or the claim is indefinite under 112(b). A general purpose computer lends sufficient structure only to basic functions of a microprocessor, such as receiving, storing and processing data².
• Invoking 112(f) does not exempt the applicant from the description, enablement and definiteness requirements of 35 U.S.C. 112(a) and (b)^2,6. See Written description requirement and Enablement requirement.

If a means-plus-function limitation lacks corresponding structure in the specification, the MPEP says rejections to consider are lack of enablement, lack of written description and indefiniteness under 35 U.S.C. 112⁶.

The broadest reasonable interpretation of a 112(f) limitation in examination is the structure, material or act described in the specification as performing the entire claimed function, and equivalents to it². A prior art element can meet the limitation only if it performs the identical function specified in the claim⁴.

The MPEP lists factors that support a conclusion that a prior art element is an equivalent of the disclosed structure⁵:

• It performs the identical function in substantially the same way and produces substantially the same results as the corresponding element disclosed in the specification⁵.
• A person of ordinary skill in the art would have recognized the two as interchangeable⁵.
• There are insubstantial differences between them⁵.

The MPEP adds that a single means claim, which recites a means-plus-function limitation as its only limitation, cannot invoke 112(f). The problem with it is that it covers every conceivable means for achieving the stated result, and such a claim should be rejected under 35 U.S.C. 112(a)².

The Federal Circuit decided in Williamson whether a term without the word "means" is governed by 112(f)³.

The court held that the term "distributed learning control module" in claims 8 to 16 was a means-plus-function claim term lacking corresponding structure. Because the patent disclosed no structure corresponding to the claimed coordinating function, it affirmed the judgment that those claims were invalid for indefiniteness3.