Before analytical chemistry was well developed, it was common to describe and define products by the process used to make them rather than trying to describe their underlying chemical structure—such patent claims were known as product-by-process claims and are still used. Such claims normally cannot be used to claim an existing product by a novel process – only the process in that context can be claimed.
The treatment of product-by-process claims is complex and varies from jurisdiction-to-jurisdiction. In some instances it has been held that a product-by-process claim only covers the product made by the process, in others it has been held that where the product is novel, making it by an alternate process will not avoid infringement. See Claims, Method Claim, Apparatus Claim.