Can the transitional phrase “consisting of” ever be interpreted as open-ended in patent claims?

This page is an FAQ based on guidance from the Manual of Patent Examining Procedure. It is provided as guidance, with links to the ground truth sources. This is information only: it is not legal advice.

Can the transitional phrase “consisting of” ever be interpreted as open-ended in patent claims?

While “consisting of” is generally considered a closed transitional phrase, there are rare circumstances where it might be interpreted as open-ended:

• Dependent claims: A dependent claim using “consisting of” might be interpreted as open-ended if its base claim uses an open-ended transition.
• Specification context: If the specification clearly indicates an open-ended interpretation, this might influence the claim’s scope.

However, these are exceptional cases. The MPEP 2111.03 emphasizes:

“The transitional phrase ‘consisting of’ excludes any element, step, or ingredient not specified in the claim.“

In general, “consisting of” is treated as a closed transition, limiting the claim to only the specified elements. Examiners and practitioners should be cautious about interpreting “consisting of” as open-ended without clear and compelling evidence from the specification or prosecution history.