In the United States, there are two tests for determining whether an accused device or process is deemed to be equivalent. Under the first test (Graver Tank & Manufacturing Co. v. Linde Air Products Co., (1950)), called the "triple identity" test, something is deemed equivalent if:
It performs substantially the same function
in substantially the same way
to yield substantially the same result.
Under the second test (Warner-Jenkinson Co. v. Hilton Davis Chem. Co. (1997)) something is deemed equivalent if there is only an "insubstantial change" between each of the elements of the accused device or process and each of the elements of the patent claim. One limitation that has been placed on this doctrine is prosecution history estoppel, which prevents a claim from being made for infringement where the difference is something that the patentee had abandoned through an amendment to the patent. It is generally considered the case that the second test builds on the first test in a doctrine of equivalents analysis.