The "doctrine of equivalents" and "prosecution history estoppel" are two interrelated equitable doctrines courts can apply in determining the reach of patents.

The doctrine of equivalents gives some leeway in interpreting the extent of a patent rather than limiting it to the exact literal meaning of the words used to describe the invention. The purpose of the doctrine is to prevent another party from taking a patented device, making minor, insignificant changes to it and thereby avoid an infringement claim.

Countering the doctrine of equivalents is prosecution history estoppel. Patent applications are typically amended one or more times before their final approval. These changes can be made for a number of reasons including to avoid claiming credit for things previously patented or publicly known ("prior art"), or because the patent examiner considers the patent too broad to be acceptable. For example, someone may submit a patent for a box stating that it is "a multi-sided container made of any substance currently known to man or which will ever be discovered." If the examiner rejects this description, the patent may later be modified to limit its scope to a "six-sided container made of cardboard." If someone was to later make a box out of plastic, patent history estoppel would bar the patentee from suing for infringement since a plastic box was covered in the original application, but was dropped in the final one which earned approval.

These two principles interact as described in Festo:

Prosecution history estoppel requires that the claims of a patent be interpreted in light of the proceedings in the PTO during the application process. Estoppel is a "rule of patent construction " that ensures that claims are interpreted by reference to those "that have been cancelled or rejected."Schriber-Schroth Co.v.Cleveland Trust Co., 311 U.S.211,220 -221 (1940). The doctrine of equivalents allows the patentee to claim those insubstantial alterations that were not captured in drafting the original patent claim but which could be created through trivial changes. When, however, the patentee originally claimed the subject matter alleged to infringe but then narrowed the claim in response to a rejection, he may not argue that the surrendered territory comprised unforeseen subject matter that should be deemed equivalent to the literal claims of the issued patent. On the contrary, "[b]y the amendment [the patentee] recognized and emphasized the difference between the two phrases [,] ....and [t]he difference which [the patentee] thus disclaimed must be regarded as material." Exhibit Supply Co.v.Ace Patents Corp., 315 U.S.126,136-137 (1942).