Claim Drafting · § 112(f)
Means-Plus-Function Claims
§ 112(f) lets you claim a function without specifying the structure — but narrows your patent to what the specification actually discloses. A tool that looks broad and often isn't.
The fundamental trade-off
A means-plus-function element says “anything that performs function X.” But § 112(f) automatically limits it to: “only the structures the specification discloses to perform X, plus their structural equivalents.” Apparent breadth, real narrowness.
The statute
35 U.S.C. § 112(f) — the text
“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.”
35 U.S.C. § 112(f) (formerly § 112 ¶ 6 before AIA)
The statute has two operative parts: (1) you mayexpress an element as “means for [function]” without specifying structure in the claim itself; and (2) in exchange, the claim shall be construed to cover only what the specification discloses plus equivalents. This is not optional — once § 112(f) is triggered, the limitation to corresponding structure is automatic.
Claim examples
What triggers § 112(f) — and what does not
Software patents
Computer-implemented means-plus-function claims
Software patents have a particularly tricky relationship with § 112(f). In WMS Gaming, Inc. v. International Game Technology (Fed. Cir. 1999), the Federal Circuit held that for computer-implemented MPF claims, the corresponding structure is the specific algorithmdisclosed in the specification — not just “a computer” or “a processor.”
Inadequate (indefinite after Williamson / WMS Gaming)
- A processor configured to perform the billing function
- A computer programmed to calculate revenue
- Means for processing the payment
Adequate corresponding structure
- A processor executing Algorithm A (described in detail in specification with pseudocode or flowchart)
- A processor running the billing module described in FIG. 5, steps 3–7
- A DSP executing the Fourier transform described in paragraphs [0054]–[0058]
Rule:For every computer-implemented function claimed in MPF format, the specification must disclose the algorithm that performs that function — described in enough detail that a POSITA can implement it. Flowcharts, pseudocode, or step-by-step textual description all qualify. “A computer” or “a processor” alone does not.
The indefiniteness trap
When MPF claims fail § 112(b) — indefiniteness
If a claim invokes § 112(f) but the specification fails to disclose adequate corresponding structure for the claimed function, the claim is indefinite under § 112(b). Under Nautilus, Inc. v. Biosig Instruments, Inc. (2014), a claim is indefinite if it fails to inform “with reasonable certainty” those skilled in the art about the scope of the invention.
After Williamson v. Citrix Online, LLC(Fed. Cir. 2015) (en banc), the Federal Circuit lowered the bar for finding § 112(f) applies — even without the word “means” — which increased the risk that nonce-word functional claims will be found indefinite for failing to disclose corresponding structure.
Williamson v. Citrix Online (Fed. Cir. 2015)
Lowered the bar for invoking § 112(f) without 'means.' 'Distributed learning control module' found to invoke § 112(f) because 'module' is a nonce word that does not connote definite structure. Claim indefinite for lack of corresponding structure.
Expanded MPF risk to any functional claim using a generic noun (module, unit, component, element, device)
Robert Bosch LLC v. Snap-on Inc. (Fed. Cir. 2015)
'Program recognition device' found NOT to invoke § 112(f) because it included 'device' with structural characteristics described. The presumption against MPF applies unless the element connotes no structure.
Clarified that structural language can rebut the Williamson presumption even for nonce-adjacent words
FAQ
Means-plus-function questions
What is a means-plus-function claim under § 112(f)?
A means-plus-function (MPF) claim is a type of patent claim element that recites a function without specifying the structure that performs it, typically using the phrase 'means for [function].' Under 35 U.S.C. § 112(f), such an element 'shall be construed to cover the corresponding structure, material, or acts described in the specification and equivalents thereof.' In other words, the claim's apparent breadth (covering any structure that performs the function) is limited by the specification — the claim only covers the specific structures described in the patent document that perform that function, plus structural equivalents. This trade-off: functional language in the claim, but structural limitation from the specification.
When does § 112(f) apply — what triggers means-plus-function interpretation?
§ 112(f) applies when a claim element uses the word 'means' (or 'step for' in method claims) combined with functional language. There is a rebuttable presumption that a claim element invoking 'means for' is governed by § 112(f). Conversely, a claim that recites specific structure (even if functional language is also used) is presumed NOT to be an MPF element — the presumption against § 112(f) can be overcome if the claim element clearly invokes functional language without any structural recitation. After Williamson v. Citrix Online, LLC (Fed. Cir. 2015) (en banc), the Federal Circuit lowered the bar for finding § 112(f) even without the word 'means' — a 'nonce word' (module, unit, device, element, component) followed by functional language may be treated as an MPF element if it does not connote sufficient definite structure to a POSITA.
What is 'corresponding structure' and why does it matter?
The corresponding structure is the specific structure (or material, or act) described in the specification that the patentee has linked to the claimed function. This is critical because it defines the scope of the MPF claim: the claim covers only that structure plus structural equivalents. If the specification does not disclose adequate structure for performing the claimed function, the claim may be indefinite under § 112(b) (Nautilus v. Biosig, 2014 — reasonable certainty standard). For computer-implemented functions, the corresponding structure must be specific algorithms, not just 'a computer programmed to perform [the function]' — WMS Gaming v. International Game Technology (Fed. Cir. 1999) established that software MPF claims require disclosure of the specific algorithm, not just the function.
What are structural equivalents in means-plus-function claims?
§ 112(f) coverage extends to the corresponding structure 'and equivalents thereof' — this is distinct from (and narrower than) the doctrine of equivalents. Structural equivalents under § 112(f) are structures that perform the identical function in substantially the same way to achieve substantially the same result (the triple-identity test), AND that are interchangeable with the disclosed structure to a person of ordinary skill at the time the patent issued (not the time of infringement). The § 112(f) equivalents analysis is more limited than the doctrine of equivalents because it applies the triple-identity test AND the 'at time of issuance' limitation, whereas the doctrine of equivalents is assessed at the time of infringement and covers broader equivalency.
Should I draft claims as means-plus-function elements?
Generally no — most patent prosecutors today avoid invoking § 112(f) because it limits claim scope to what is specifically described in the specification. If a broad structural claim can be drafted, it will typically provide broader coverage. However, MPF claiming can be strategically useful when: (1) the invention is defined by a function that would be difficult to describe structurally; (2) the patentee wants to ensure the claim covers all structural ways to perform that function (at issuance) without being limited to one specific structure; (3) drafting claims for technology that is rapidly evolving. If you use MPF, ensure the specification contains a full, explicit description of the corresponding structure linked to the function — missing or inadequate disclosure of corresponding structure will make the claims indefinite.