Introduction
35 U.S.C. § 112(f) is an important provision of U.S. patent law concerning means-plus-function and step-plus-function claim limitations. It allows an applicant to express a claim element in terms of the function it performs rather than reciting all of the corresponding structure, material, or acts in the claim.
However, using § 112(f) has an important consequence: the limitation is construed to cover the corresponding structure, material, or acts described in the specification and equivalents thereof.
Understanding § 112(f) is important for both patent prosecution and Patent Bar examination preparation.
What Does 35 U.S.C. § 112(f) Provide?
Section 112(f) provides, in relevant part, that an element in a combination may be expressed as a means or step for performing a specified function without reciting the structure, material, or acts supporting that function.
When this occurs, the claim is construed to cover the corresponding structure, material, or acts described in the specification and their equivalents.
This provision is sometimes called the means-plus-function provision.
The USPTO’s primary examination guidance is found in MPEP § 2181.
Why Is § 112(f) Important?
Section 112(f) creates a balance between claim drafting and disclosure.
An applicant can use functional language in a claim without listing every structural detail. In exchange, however, the scope of that limitation is tied to the corresponding structure, material, or acts disclosed in the specification and equivalents.
For example, consider:
“A device comprising a means for detecting temperature.”
The phrase “means for detecting temperature” may invoke § 112(f).
The examiner must then look to the specification to determine what structure is disclosed as performing the claimed temperature-detection function.
If the specification discloses a particular sensor or other structure that performs the entire claimed function, that disclosed structure and its equivalents may define the scope of the § 112(f) limitation.
The Three-Part USPTO Analysis
The USPTO instructs examiners to use a three-prong analysis when determining whether a claim limitation invokes § 112(f).
Prong 1: Means, Step, or Generic Placeholder
The claim limitation must use:
- the term “means”;
- the term “step”; or
- a generic placeholder or “nonce” term that serves as a substitute for “means.”
Examples of possible generic placeholders include terms that do not themselves convey sufficient structural meaning.
The analysis is based on the claim language and the circumstances of the particular case.
Prong 2: Functional Language
The means, step, or generic placeholder must be associated with functional language.
The claim may use the traditional phrase:
“means for performing [function]”
But other language may also express a function, including phrases such as “configured to” or “so that,” depending on the claim language and context.
Simply using the word “means” without a specified function does not automatically invoke § 112(f).
Prong 3: No Sufficient Structure, Material, or Acts
The claim limitation must lack sufficient structure, material, or acts for performing the claimed function.
If the claim itself recites sufficient structure for performing the function, § 112(f) may not apply.
Thus, the examiner considers the claim as a whole rather than simply searching for the word “means.”
Example of a Means-Plus-Function Limitation
Consider:
“A system comprising a processor and means for detecting an unauthorized access attempt.”
The phrase “means for detecting an unauthorized access attempt” may invoke § 112(f).
The examiner would then examine the specification to identify the corresponding structure that performs the entire detecting function.
If the specification identifies a particular hardware arrangement or appropriately disclosed computer implementation, that disclosure becomes important in construing the claim limitation.
The Presumption When “Means” Is Used
When a claim explicitly uses the word “means” together with functional language, there is generally a presumption that § 112(f) applies.
For example:
“means for transmitting data”
is presumptively a means-plus-function limitation.
The presumption can be overcome if the claim itself includes sufficient structure, material, or acts for performing the claimed function.
The USPTO describes this analysis in MPEP § 2181.
What If the Claim Does Not Use the Word “Means”?
The absence of the word “means” does not necessarily end the inquiry.
A claim limitation using a generic placeholder can still invoke § 112(f).
For example, a term such as:
“module configured to process the received data”
may require an analysis of whether “module” is functioning as a generic placeholder and whether the claim supplies sufficient structure.
The Federal Circuit’s decision in Williamson v. Citrix Online, LLC, 792 F.3d 1339 (Fed. Cir. 2015), is an important authority concerning functional claim language and generic placeholders.
The USPTO discusses this principle in MPEP § 2181.
Corresponding Structure Must Be Found in the Specification
Once § 112(f) applies, the examiner must determine the corresponding structure, material, or acts disclosed in the specification.
This is one of the most important concepts in § 112(f) practice.
The specification should provide an adequate disclosure identifying what performs the claimed function.
For example, if the claim recites:
“means for filtering a received signal”
the specification should disclose the structure that performs the entire claimed filtering function.
The corresponding structure is not simply any structure mentioned somewhere in the specification. The specification or prosecution history must provide an appropriate relationship between the disclosed structure and the claimed function.
The USPTO discusses this requirement in MPEP § 2181.
What Happens if No Corresponding Structure Is Disclosed?
If a claim invokes § 112(f), but the specification does not disclose sufficient corresponding structure, material, or acts for performing the claimed function, the claim may be rejected as indefinite under 35 U.S.C. § 112(b).
This is an important prosecution issue.
The applicant cannot simply rely on the functional language in the claim while providing no adequate disclosure of what performs the function.
The USPTO explains that § 112(f) does not eliminate the requirements of § 112(a) or § 112(b).
§ 112(f) and Written Description
A § 112(f) limitation must also be considered under 35 U.S.C. § 112(a).
The specification must provide sufficient disclosure to demonstrate that the inventor had possession of the claimed invention and must satisfy the applicable enablement requirements.
Thus, invoking § 112(f) does not provide an exemption from the written description requirement.
The USPTO explains that an inadequate disclosure supporting a § 112(f) limitation can raise both indefiniteness and written-description issues.
Computer-Implemented § 112(f) Limitations
Computer-implemented inventions present an especially important § 112(f) issue.
Suppose a claim recites:
“a processor means for determining whether the transaction is fraudulent.”
If § 112(f) applies, simply stating that a general-purpose computer performs the function may not be enough.
For a computer-implemented means-plus-function limitation, the specification generally must disclose an algorithm for performing the claimed computer function.
The USPTO discusses this requirement in MPEP § 2181 and cites Federal Circuit decisions including In re Aoyama, Aristocrat Technologies Australia Pty Ltd. v. International Game Technology, and WMS Gaming, Inc. v. International Game Technology.
Why an Algorithm Can Be Important
Consider the following claim:
“A computer comprising means for determining whether a received transaction is fraudulent.”
If § 112(f) applies, the specification should disclose how the computer performs the claimed function.
A statement such as:
“The computer determines whether the transaction is fraudulent.”
may merely restate the claimed function rather than provide an algorithm or other adequate corresponding structure.
The specification should provide sufficient disclosure of the procedure or algorithm used to perform the claimed function.
§ 112(f) Does Not Mean “Anything That Performs the Function”
A common misunderstanding is that a means-plus-function limitation covers every possible structure capable of performing the claimed function.
That is generally incorrect.
Under § 112(f), the claim is construed to cover the corresponding structure, material, or acts described in the specification and equivalents thereof.
Consequently, § 112(f) can produce a narrower claim construction than a conventional structural limitation.
The USPTO expressly instructs examiners that the broadest reasonable interpretation of a § 112(f) limitation is tied to the disclosed corresponding structure, material, or acts and equivalents.
Examiner’s Obligation During Examination
When an examiner determines that § 112(f) applies, the USPTO instructs the examiner to make the record clear.
The examiner should identify:
- that § 112(f) is being applied;
- the claimed function; and
- the corresponding structure, material, or acts identified in the specification.
This allows the applicant to understand the claim construction being used during examination.
The USPTO provides specific guidance on maintaining a clear prosecution record in MPEP § 2181.
Applicant’s Response to a § 112(f) Determination
If the applicant disagrees with the examiner’s determination that § 112(f) applies, the applicant may present arguments explaining why the claim limitation contains sufficient structure.
Alternatively, the applicant may amend the claim to clarify the structural limitation.
For example, instead of:
“means for controlling the valve”
the applicant might recite specific structure:
“a controller coupled to the valve and configured to control the valve based on the detected pressure.”
Whether the amended language avoids § 112(f) depends on the particular claim language and the circumstances.
The USPTO provides guidance concerning applicant responses in MPEP § 2181.
Means-Plus-Function vs. Ordinary Functional Claim Language
Not every functional claim limitation invokes § 112(f).
For example, a claim may recite:
“a temperature sensor configured to detect a temperature.”
The term “temperature sensor” may provide structural meaning.
The fact that the sensor is described by its function does not automatically convert it into a means-plus-function limitation.
The examiner must determine whether the claim term denotes sufficient structure to a person of ordinary skill in the art.
The USPTO discusses this distinction in MPEP § 2181.
Step-Plus-Function Limitations
Section 112(f) also applies to certain step-plus-function limitations in method claims.
For example:
“performing a step for determining the authentication status of the user.”
The examiner must determine whether the limitation invokes § 112(f) and, if so, identify the corresponding acts disclosed in the specification.
The USPTO explains that § 112(f) can apply to method claims as well as apparatus claims.
Important Patent Prosecution Example
Assume a claim states:
“A communication apparatus comprising a means for encrypting a data packet.”
The specification discloses an encryption processor and describes an algorithm for encrypting the data packet.
During examination, the examiner determines that the limitation invokes § 112(f).
The examiner should identify the § 112(f) limitation and consider the corresponding structure disclosed in the specification.
If the examiner identifies no adequate corresponding structure, a § 112(b) indefiniteness issue may arise.
The applicant may respond by:
- pointing to the disclosed corresponding structure;
- explaining why the disclosed structure satisfies § 112(f);
- amending the claim; or
- addressing any related § 112(a) or § 112(b) issues.
Important Patent Bar Rules
For Patent Bar examination purposes, remember these principles:
Rule 1: “Means for” + function generally creates a presumption that § 112(f) applies.
Rule 2: The presumption can be overcome by sufficient structure in the claim.
Rule 3: A claim without the word “means” can still invoke § 112(f) if a generic placeholder is used.
Rule 4: § 112(f) is determined on a limitation-by-limitation basis.
Rule 5: The specification must disclose corresponding structure, material, or acts.
Rule 6: The corresponding structure must perform the entire claimed function.
Rule 7: A computer-implemented § 112(f) limitation generally requires disclosure of an algorithm for the claimed computer function.
Rule 8: Failure to disclose adequate corresponding structure can result in a rejection under § 112(b).
Rule 9: § 112(f) does not eliminate the requirements of § 112(a).
Rule 10: The examiner should make the § 112(f) claim construction clear in the prosecution record.
Patent Bar Practice Question
Question:
A claim recites:
“A system comprising a means for determining whether a received signal contains an error.”
The specification describes a particular error-detection circuit that performs the entire claimed function.
The examiner determines that the limitation invokes § 112(f).
Which statement is most accurate?
A. The limitation covers every conceivable device capable of detecting an error.
B. The limitation is interpreted according to § 112(f), including the corresponding structure disclosed in the specification and equivalents thereof.
C. The limitation is automatically indefinite because the word “means” is used.
D. Section 112(f) applies only to method claims.
Answer: B.
The use of “means” with functional language can invoke § 112(f). When § 112(f) applies, the claim is construed to cover the corresponding structure, material, or acts described in the specification and equivalents thereof.
Conclusion
35 U.S.C. § 112(f) permits an applicant to claim an element by the function it performs, but the resulting claim limitation is tied to the corresponding structure, material, or acts disclosed in the specification and equivalents thereof.
For patent prosecution, the most important questions are:
Does the limitation invoke § 112(f)?
What is the claimed function?
What corresponding structure, material, or acts are disclosed in the specification?
Does that disclosure perform the entire claimed function?
For a computer-implemented function, is an adequate algorithm disclosed?
Understanding these questions helps applicants and practitioners draft claims, respond to Office Actions, and analyze § 112(f) issues more effectively.
References
The following authorities are particularly useful when studying § 112(f):
- 35 U.S.C. § 112(f) — Means-plus-function and step-plus-function limitations.
- 35 U.S.C. § 112(a) — Written description and enablement.
- 35 U.S.C. § 112(b) — Definiteness.
- MPEP § 2181 — Identifying and interpreting a § 112(f) limitation.
- MPEP § 2182 — Corresponding structure, material, or acts.
- MPEP § 2183 — Treatment of § 112(f) limitations during examination.
- MPEP § 2184 — Relationship of § 112(f) to other requirements.
- MPEP § 2185 — Claim interpretation involving § 112(f).
- Williamson v. Citrix Online, LLC, 792 F.3d 1339 (Fed. Cir. 2015).
- In re Donaldson Co., 16 F.3d 1189 (Fed. Cir. 1994) (en banc).
- In re Aoyama, 656 F.3d 1293 (Fed. Cir. 2011).
- WMS Gaming, Inc. v. International Game Technology, 184 F.3d 1339 (Fed. Cir. 1999).
The USPTO also provides dedicated § 112(f) examiner form paragraphs and guidance, which are useful for understanding how these issues appear in an Office Action.
Educational disclaimer: This article provides general educational information about U.S. patent prosecution. It is not legal advice and does not create an attorney-client relationship.