When a patent is applied for, the inventor must include a specification, which is a written description of the invention, the manner and process by which the invention is made, and the manner and process by which the invention is used. A 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. Each claim must conform to the description of the invention set out in the specification.
The doctrine of equivalents represents a certain flexibility in the rule that the language of a patent claim defines the scope of patent protection. 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 alleged infringer if the changes in the allegedly infringing invention are so slight that the allegedly infringing invention can be considered to be the equivalent of 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.