The Supreme Court’s decision in Merrill v. Yeomans is widely and correctly viewed as a watershed case in which the Court first announced that the “distinct and formal claim” has “primary importance” in determining “precisely what it is that is patented.” But the reasoning of the case relied on a pernicious myth that the patent system of the era contained such “well-settled rules” as to “leave no excuse for ambiguous language or vague descriptions” in textual descriptions of inventions. Merrill’s myth has had unfortunate consequences, with courts occasionally placing unrealistic demands upon inventors—and, really, inventors’ attorneys—to write claims with a degree of clarity that is often impossible given the technological and legal knowledge at the time of patenting.

John F. Duffy, The Myth of Well-Settled Rules in Merrill v. Yeomans, 71 Case Western Reserve Law Review, 667–698 (2020).