Understanding Functional Claims: What is an Example of a Function Claim?

December 12, 2025 •

4 min read

In patent law, a function claim describes an invention based on its performance rather than its structure. The U.S. Patent and Trademark Office allows this under specific rules, known as 'means-plus-function' claiming. A practical example of a function claim is essential for understanding this complex legal strategy.

Quick Summary

Functional claims in patents describe an invention's purpose, using phrases like 'means for' and are interpreted narrowly based on the patent's specification. In food and supplements, structure/function claims describe a nutrient's role in the body, such as 'calcium builds strong bones'.

Key Points

Patent Claim Example: A 'means for attaching the legs to the top' of a table is a classic patent example of a function claim.
Defined by Function, Not Structure: Functional claims describe an invention by what it does, while structural claims describe its physical components.
Interpretation of Patent Claims: Functional patent claims are limited in scope to the specific structures disclosed in the patent's written description and their equivalents.
Dietary Supplement Claims: A structure/function claim for a supplement, like 'calcium helps build strong bones,' is distinct from a disease claim.
Legal Risk: If a patent fails to disclose a corresponding structure for a functional limitation, the claim can be deemed indefinite and invalid.
Other Functional Language: Phrases like 'configured to' and 'module for' can also be interpreted as functional language in patents.

What is a Functional Claim?

A functional claim limitation describes an element of an invention by what it does, rather than detailing its specific physical structure. This approach is often used in complex or evolving technological fields, such as software or complex mechanical devices, where an inventor wants to cover a broad range of potential structures that could perform the same task. The most common form of a functional claim in U.S. patent law is the 'means-plus-function' claim, governed by 35 U.S.C. § 112(f).

Example of a Function Claim in a Patent

A classic and easy-to-understand example of a function claim can be found in the context of a simple mechanical device. Imagine a patent for a backscratching tool. A structural claim would describe the tool using specific parts, like "a 1-3 foot long stick and a brush comprising stiff bristles attached to one end". A functional claim would describe the same device very differently:

An apparatus, comprising an extension means for extending the reach of a human user's hand, and a back skin stimulating means attached to a distal end of said extension means.

In this example, the phrases "extension means" and "back skin stimulating means" are functional language. They define the invention by its purpose, not its parts. The legal interpretation of this claim would be limited to the specific structure disclosed in the patent's written description (e.g., the stick and brush) and any equivalents thereof.

Other Functional Language

Beyond the classic 'means for' language, patent law also recognizes other functional phrases that can trigger similar interpretations. These can include terms like:

Module for
Device for
Mechanism for
Configured to
Adapted to

However, the use of such language does not automatically make a claim functional. Courts will examine whether the claim recites enough structural detail to avoid being interpreted under the stricter rules of § 112(f).

The Function Claim in Practice: Pros and Cons

Using functional claiming is a strategic choice for patent applicants. It offers the potential for broader claim coverage, but also carries significant risks if not executed perfectly. The primary trade-off is between claiming breadth and definitional clarity.

Advantages of Functional Claims

Broad Scope: They can theoretically cover future technologies or structural variations that perform the same function, provided they are equivalent to the disclosed structure.
Simplified Drafting: It can be easier to draft a claim focusing on an invention's purpose, especially for complex software or electrical systems where the implementation details are numerous and complex.

Disadvantages of Functional Claims

Narrow Interpretation: Despite appearing broad on its face, a court will interpret a functional claim narrowly, limiting it to the specific structures disclosed in the patent's specification and their equivalents.
Indefiniteness Risk: If the patent specification does not clearly disclose a corresponding structure for performing the claimed function, the claim can be found invalid as 'indefinite'.

Comparison: Structural vs. Functional Claims


Feature	Structural Claims	Functional (Means-Plus-Function) Claims
Definition	Describes an invention's physical components or arrangements.	Describes an invention by what it does or its purpose.
Example	"A stick with a bristled brush on the end."	"A means for extending a user's reach."
Scope	Limited to the specific structure claimed.	Potentially broader, covering the disclosed structure and its equivalents.
Interpretation	Typically interpreted according to the plain and ordinary meaning of the words.	Limited by the structure described in the patent specification.
Risk	Can be easier to circumvent by designing a non-infringing structural variation.	Risk of being found invalid for indefiniteness if the corresponding structure is not disclosed.
Primary Use	Most common for straightforward mechanical or structural inventions.	Strategic choice for complex technologies like software or mechanics.

The Function Claim in Dietary Supplements

Outside of intellectual property, the term "function claim" is used differently in the regulation of food and dietary supplements by agencies like the FDA in the US. A structure/function claim describes how a nutrient or dietary ingredient affects the normal structure or function of the human body. Critically, these claims cannot imply that the product can diagnose, treat, cure, or prevent any disease.

Example of a Food/Supplement Function Claim

Consider a product containing calcium. A permissible structure/function claim is:

"Calcium helps build and maintain strong bones."

This statement focuses on the positive role of calcium in a healthy body's function. An impermissible, or disease, claim would be something like:

"Calcium supplement prevents osteoporosis."

This claim links the product to the prevention of a disease and would require drug-level approval.

Conclusion

Understanding what a function claim is requires distinguishing between its use in patent law and its application in dietary supplement marketing. In patents, it defines an invention by its performance, using 'means-plus-function' language, and is interpreted narrowly based on the disclosed specification. A concrete example of a function claim in this field might be a "means for extending" a user's reach. In the context of supplements, a structure/function claim describes how a nutrient supports a normal body function, like "calcium builds strong bones," and must be carefully worded to avoid implying disease treatment. In both domains, correctly articulating the claim is crucial for legal compliance and protection.

Frequently Asked Questions

A 'means-plus-function' claim is a type of functional patent claim that uses the phrase 'means for' to describe an invention's element by what it does, not its structure. It is interpreted based on the corresponding structure disclosed in the patent's specification.

The primary risk is that the claim could be found invalid for indefiniteness if the patent's written description does not sufficiently disclose a structure that performs the claimed function. The interpretation of these claims can also be much narrower than their plain language suggests.

A function claim describes an invention in terms of its purpose or what it accomplishes, while a structure claim defines the invention by its physical components. Functional claims are interpreted more narrowly than their wording suggests, based on the patent's specification.

Yes, a functional claim can cover future technology, but only if that technology is considered an 'equivalent' to the specific structure disclosed in the patent specification. This can make the claim potentially broad, but the definition of 'equivalent' can be a point of contention.

For dietary supplements and food, a structure/function claim describes how a nutrient or ingredient affects the normal structure or function of the human body, such as 'Calcium builds strong bones.' It must not claim to treat, mitigate, or prevent a disease.

Under 35 U.S.C. § 112(f), a court will interpret a functional claim to cover only the specific structure disclosed in the patent's specification for performing the function, along with its equivalents. This makes the claim's scope dependent on what is described in the patent text.

An impermissible disease claim links a product to treating, curing, or preventing a disease. For example, claiming that a supplement 'cures the common cold' is a disease claim, whereas 'supports the immune system' is a proper structure/function claim.