Graver Tank & Manufacturing Co. v. Linde Air Products Co., 339 U.S. 605 (1950), was an important United States Supreme Court decision in the area of patent law, establishing the propriety of the doctrine of equivalents, and explaining how and when it was to be used. Facts The plaintiff Linde Air Products Co. owned a patent for an electric welding process, and sued defendants including the Graver company for infringing the patent. The defendants asserted that they were not infringing the patent because the patented welding process used a welding composition made of alkaline earth metal silicate and calcium fluoride (usually expressed as silicates of calcium and magnesium), while the purported infringers substituted a similar element, manganese, for the patentee's magnesium. The United States district court found infringement, and the Court of Appeals affirmed the infringement claim. Issue The Supreme Court agreed to review the case, limited to the question of whether the substitution of a similar material not claimed in the patent itself would save the defendants from being held liable for infringements. Result The Court, in an opinion written by Justice Robert Jackson, raised the doctrine of equivalents. It noted that if another party could use a process exactly the same as one that is patented, but escape infringement by making some obvious substitution of materials, it would deprive the patentee of the exclusive control meant to come with a patent. This would undermine the profitability of the patent, which would go against the policy of encouraging inventors to invent by giving the opportunity to profit from the labor of invention. The Court also outlined how the doctrine should be used, noting that "what constitutes equivalency must be determined against the context of the patent, the prior art, and the particular circumstances of the case." The Court laid out two possible tests to determine equivalency. Under the first of these (which has since come to be known as the "triple identity" test), something is deemed equivalent if: It performs substantially the same function in substantially the same way to obtain the same result. Under the second test, something is deemed equivalent if there is only an "insubstantial change" between each of the features of the accused device or process and the patent claim. In this case, the Court gave particular weight to the determination of "whether persons reasonably skilled in the art would have known of the interchangeability