The en banc Federal Circuit in Williamson v. Citrix struck out against what it called “a proliferation of functional claiming,” by easing the burden of showing when a patent claim should be construed under 35 USC Section 112(f). This section limits the scope of a functional claim to structure disclosed in the specification – also known as a “means-plus-function” claim. This holding is significant because claims subject to Section 112(f), especially when not intended as such by the draftsperson, are often construed more narrowly or invalidated more easily than if Section 112(f) were not implicated.

Functional claiming refers to a patent claim that seeks to claim an apparatus, system or device, e.g., a computer network, not by what it is, but by the function it performs. These claims can present a challenge because it can be extremely difficult to determine what sorts of limits are placed on the functional language expressed in the claim. What tangible things that perform this same function would infringe the claim? Are any of those things already in the prior art? When either of these questions cannot be answered the claim is invalid, because the boundaries of a patent claim must be ascertainable to the public in order to comply with statutory requirements. It is however very difficult, generally speaking, to show that a claim is invalid for indefiniteness under Section 112(b). Needless to say, this status quo for a duly issued US patent has favored the patentee who wishes to assert its patent among a diverse pool of seemingly unrelated defendants.

Prior to Williamson, Section 112(f) was invoked (or not) during claim construction depending, in large part, on whether “means for” or “means” appeared in the claim. This standard originated from the same words appearing in the statute: “[a]n element in a claim for a combination may be expressed as a means . . . for performing a specified function.” The Williamson court, rejecting this standard, emphasized that the appropriate test should focus more on whether there is “sufficiently definite structure” in the claim and less on whether the word “means” is used.

Functional claims subject to Section 112(f) should be easier to deal with, in many instances, than a functional claim free from the restrictions imposed by the statute. When Section 112(f) applies, the claim can be found indefinite for simply failing to disclose any structure that can perform the claimed function – regardless of how well known is the technology that can perform the function. And even if the claim survives, there is a good chance it will be construed far more narrowly than if Section 112(f) were not invoked, especially when the patent specification describes few alternative embodiments.