Patent Infringement – Doctrine of Equivalents

As previously described, when a patent is applied for, the inventor must include a specification, including the written description of the invention. The specification concludes with a claim or claims that particularly point out and distinctly claim the subject matter that the applicant asserts to be the invention to be patented. One of the purposes of claims is to put the public on notice as to what inventions have been patented and may not be patented again. Therefore, the language of claims is exceedingly important in a patent application because, in the event of a patent dispute, the language of the claims will define the scope of the patent protection.

Ordinarily, even if patentable, any part of the invention that is not included in the claims will not receive patent protection. Generally, each claim must conform to the description of the invention set forth in the specification.

In the event a patent is infringed, the patentee may sue under theories of literal infringement or under a theory of non-literal infringement also referred to as the doctrine of equivalents. Literal infringement means each and every element is met by the infringing device. This theory is often harsh in that it requires matching every element of the invention as claimed with a corresponding element on the infringing product. For example, if a patentee describes an invention in terms of two wheels and the infringing product includes two casters, the product may not infringe, even the wheels and casters are very similar.

The doctrine of equivalents represents a certain flexibility in evaluating infringement by evaluating the language of a patent claim. Under the doctrine of equivalents, if an invention that is claimed to infringe a patent does not come within the language of a patent claim and, thus, seemingly does not infringe the patent, a patentee might still prevail against an infringer if the changes in the infringing product are so slight that the elements of the infringing product should be considered to be equivalent to the patented invention. This provides patent protection against devices that make insubstantial changes that the inventor may not have foreseen and, thus, did not include within his or her claim.


There are certain limits to the application of the doctrine of equivalents. During the patent process, part of a claim may be found to have already been patented and thus does not qualify for a patent. A patent applicant will accordingly amend the subject matter of the claim so that there is no overlap with existing patents. In such a case, a patentee may not later seek protection of the subject matter that was removed from his or her claims by invoking the doctrine of equivalents. This limitation on the doctrine of equivalents is known as “prosecution history estoppel.” The doctrine of equivalents may apply, depending upon the particular circumstances, to amendments made to a claim that expanded the subject matter of the original claim rather than relinquished subject matter. The doctrine of equivalents may not be invoked in order to claim protection of an invention that is in the public domain.

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