United States patent law bars the grant of a valid patent if the invention claimed has been "on sale" in the United States for more than one year before the filing of the patent application in question. The United States Supreme Court in Pfaff v. Wells Electronics Inc. considered what had to be established to determine whether this test was met.

The Court stated that the "on sale" bar applies when the invention is the subject of a commercial offer for sale and the invention is ready for patenting. The invention is considered ready for patenting when the invention is reduced to practice before the critical date or when there is a description of the invention or drawings illustrating the invention that are "sufficiently specific to enable a person skilled in the art to practice the invention".

The Court found that Pfaff's actions more than one year before the filing of the application constituted a commercial offer, rather than experimental testing, and the drawings Pfaff sent to the manufacturer disclosed the invention in sufficient detail to enable one skilled in the art to practice the invention. The Court considered that this triggered the "on sale" bar and thus affirmed the appeal court's ruling that the patent was invalid.