Infringement of a utility patent is determined by comparing the accused article or method with the claims of the patent. If the article or method meets each element of at least one of the patent’s claims, then it infringes that patent. If the article or method meets the literal words of the claim, that claim is said to be literally infringed. However, even if the article or method does not literally meet each element of a claim, it can still infringe that claim under the Doctrine of Equivalents. The Doctrine of Equivalents is a judicially-created doctrine that prevents unimportant deviations from the literal language of the claims from avoiding infringement. Thus even if an accused article or method literally lacks a claim element, if it is not substantially different from the claimed invention, it may still infringe under the doctrine of equivalents.
In determining what is an infringing equivalent to a claim element, it is often necessary to refer to the patent’s specification. It may also be necessary to consider the prosecution history of the application – the statements and arguments made by the patent applicant to obtain the patent. The Doctrine of Equivalents cannot be used to recapture subject matter excluded by the patent applicant’s statements or actions before the Patent Office – this is known as the Doctrine of Prosecution History Estoppel.