A more thorough explanation:

The doctrine of equivalents is a legal principle that allows a patent holder to claim infringement even if every element of their invention is not present in an allegedly infringing product. The purpose of this doctrine is to prevent an infringer from making minor changes to a patented invention and still benefiting from its functionality.

For example, if a patent holder invents a new type of screwdriver with a unique handle and a specific type of head, an infringing product that uses a different handle but the same type of head may still be considered an infringement under the doctrine of equivalents.

The doctrine of equivalents is determined by a "triple identity" test, which requires that the accused product or process performs substantially the same function in substantially the same way to obtain the same result as the patented invention. This means that each individual element of a claim must be analyzed to determine if it is present in the accused product or process.

For example, if a patent holder invents a new type of battery with a specific chemical composition, an infringing product that uses a different chemical composition but still produces the same amount of power may still be considered an infringement under the doctrine of equivalents.

The doctrine of equivalents is evaluated at the time of infringement, not at the time the patent was issued. This means that subsequent changes in technology may challenge the definition of previously essential elements or limitations present in the original invention.