The requirement to describe the best mode is a separate and in addition to providing a written description and an enabling disclosure. The purpose of the provision is to prevent inventors from obtaining patents while at the same time concealing preferred embodiments of the inventions they have conceived. [62]

If at the time of filing a patent application in the United States, or in a foreign country from which priority is claimed, [63] the inventor knew of an embodiment for carrying out the invention that forms the subject of the claims of his patent as granted that is better than any of the embodiments described in the application the patent or at least the claims covering the embodiment in question will be invalid for failure to comply with the best mode requirement. [64] Unlike most of patent law, however, the test here includes a subjective element. The requirement to disclose the best mode is not absolute. It is to disclose what the inventor contemplated as the best mode. Thus to succeed in challenging a patent on this ground it is necessary to show that the inventor did know of the mode in question at the time of filing the application. However, it is irrelevant whether the patent applicant was in fact the inventor of that best mode if he in fact knew of it. [65] In Chemcast Corp. v. Arco Industries [66] the court applied a two-part test for determining whether the best mode requirement was met:

Thus, the first step involves a subjective test and the second step an objective one.