Linde Air Products Co. (Linde) (plaintiff) owned a patent for Unionmelt Grade 20, a flux composition used in welding. Though the patent claimed alkaline earth metal silicate and calcium fluoride as ingredients, Unionmelt actually contained calcium and magnesium silicates. Linde sued Graver Tank & Manufacturing Co. (Graver) (defendant), claiming Graver’s flux composition, Lincolnweld 660, infringed the patent. Though the uses and welding quality produced by the compositions were largely the same, Lincolnweld did not contain magnesium like Unionmelt. Instead, Lincolnweld contained manganese, which is not an alkaline earth metal. At trial, chemists skilled in the art testified that manganese and magnesium act similarly in chemical reactions, and that, for purposes of the patent, manganese could be considered an alkaline earth metal. Additionally, evidence established that prior art taught the substitution of manganese silicate in welding fluxes. Graver produced no evidence demonstrating that Lincolnweld was discovered or developed based on independent research or experimentation. The trial court concluded that Lincolnweld and Unionmelt were substantially the same in use and quality of results, and that Lincolnweld was equivalent to Unionmelt for purposes of metal welding. Thus, the trial court held that the patent claims were infringed. On appeal before the United States Supreme Court, the Court considered the applicability of the doctrine of equivalents to the flux claims.