What is the difference between comprising and consisting of in patent claims?

In a U.S. patent claim, the transitional phrase connecting the preamble to the body determines whether the claim is open to additional, unrecited elements. Three phrases dominate practice: comprising, consisting of, and consisting essentially of. Each creates a different legal scope, and choosing the wrong one can either leave a gap that a competitor can exploit or sacrifice breadth that you legitimately earned.

Comprising (open-ended)

"Comprising" is inclusive and open-ended, meaning it does not exclude additional, unrecited elements or method steps beyond those named in the claim. The phrase is synonymous with "including," "containing," and "characterized by."

The Federal Circuit confirmed this in Genentech, Inc. v. Chiron Corp., 112 F.3d 495 (Fed. Cir. 1997), holding that "comprising" means "the named elements are essential, but other elements may be added and still form a construct within the scope of the claim."

Practical effect. A claim reading "A composition comprising A, B, and C" is infringed by a product that contains A, B, C, and D. The accused product must include every recited element, but having additional unrecited elements does not avoid infringement.

Consisting of (closed)

"Consisting of" excludes any element, step, or ingredient not specifically recited in the claim. The closed construction dates to In re Gray, 53 F.2d 520 (CCPA 1931), which defined the phrase as closing the claim to the inclusion of materials other than those recited.

A narrow exception permits trace impurities ordinarily associated with a recited ingredient; those incidental impurities do not open the claim to unrecited components.

Practical effect. A claim reading "A composition consisting of A, B, and C" is not infringed by a product containing A, B, C, and D. The extra element D takes the product outside the claim unless D qualifies as an ordinarily associated impurity.

One important nuance: when "consisting of" appears inside a clause within the body of a claim rather than as the main transitional phrase after the preamble, the closure applies only to that specific clause or element group, not to the entire claim. Practitioners rely on this distinction when drafting a claim with open overall scope but a closed Markush group for one particular element (for example, "selected from the group consisting of A, B, and C").

Consisting essentially of (middle ground)

"Consisting essentially of" occupies a middle ground between open and closed claim language, permitting the recited materials or steps plus any additional elements that do not materially affect the basic and novel characteristics of the claimed invention.

An applicant who contends that additional elements in the prior art are excluded by "consisting essentially of" bears the burden of showing that those additional elements would materially change the characteristics of the claimed invention. Absent a clear indication in the specification of what the basic and novel characteristics are, "consisting essentially of" may be construed as equivalent to "comprising."

At a glance

When to use which

Comprising is the default for most claims. It gives the broadest scope and makes it harder for competitors to design around by adding unrecited elements. The trade-off is that a broad comprising claim may read on prior art that a narrower claim would clear.

Consisting of is appropriate when the invention IS a specific combination and nothing else. It appears frequently in Markush groups ("selected from the group consisting of..."), tightly defined chemical formulations, and method steps that must form a closed set. The trade-off is vulnerability to design-arounds: a competitor who adds even one additional ingredient or step steps outside the claim.

Consisting essentially of is a specialized tool used primarily in chemistry and pharmaceutical claims, where minor additives (stabilizers, trace solvents, fillers) that real-world manufacturing produces should not destroy coverage, but the essential core composition should remain fixed. To make "consisting essentially of" enforceable as intended, define the basic and novel characteristics of the invention clearly in the specification.

Practical notes

• Use comprising as the default. Breadth is easier to narrow during prosecution than narrowness is to widen. Draft the broadest independent claim with comprising, then add narrowing dependent claims.
• Layer comprising claims with consisting of sub-claims. A common pattern: claim 1 uses "comprising" for maximum scope; a dependent claim narrows to a specific formulation using "consisting of." This gives both broad reach and a fallback that precisely captures the preferred embodiment.
• Watch for "consisting of" inside the body. When a Markush group within a claim reads "selected from the group consisting of A, B, and C," the closure covers only that group. The main preamble transition, not the internal phrase, controls the rest of the claim's openness.
• Define basic and novel characteristics before using "consisting essentially of." If the specification never identifies those characteristics, an examiner or court may treat the phrase as equivalent to "comprising," erasing the intended middle-ground protection.
• Design-around risk is asymmetric. Competitors can escape a "consisting of" claim by adding any element. They cannot escape a "comprising" claim merely by adding elements. Align your transitional phrase choice with the specific business risk you are guarding against.