§112(f) allows a patent applicant to claim an element by the function it performs, however functionally claimed elements using §112(f) are limited to what is disclosed in the specification and equivalents.  The classic form of a 112(f) claim element is:

means for + [GERUND]

for example “means for fastening.”  However, many prosecutors, attempting to free themselves from the bounds of the structures disclosed in their specifications, attempt to claim an element by the function it performs.]

However, it is not possible to write a truly functional claim that avoids 112(f).  A claim element that does not use the word “means” is presumed to not invoke §112(f), but if it fails to recite “sufficiently definite structure” or recites “function without reciting sufficient structure for performing that function,” then that element will nevertheless be construed under 112(f).  The standard is whether the words of the claim are understood by persons of ordinary skill in the art to have a sufficiently definite meaning as the name for structure.  Williamson v. Citrix Online, LLC, 792 F.3d 1339, 1346 [115 USPQ2d 1105] (Fed. Cir. 2015).

If the functional term is found to have  a sufficiently definite meaning as the name for structure, the prosecutor has succeeded in broadly claiming the invention, while if it is not found to have a sufficiently definite structural meaning, then 112(f) applies, whether or not that is what the prosecutor intended.

While converting the gerund “fastening” in the example above, to “fastener” would seem to be safe, one cannot be completely certain that the selected term conveys a sufficiently definite structural meaning. See Advanced Ground Info. Sys., Inc. v. Life360, Inc., 119 USPQ2d 1526 (Fed. Cir. 2016)(symbol generator) ; Media Rights Techs., Inc. v. Capital One Fin. Corp., 800 F.3d 1366, [116 USPQ2d 1144, 1148] (Fed. Cir. 2015)(compliance mechanism); Williamson v. Citrix Online, LLC, 792 F.3d 1339 [115 USPQ2d 1105, 1112 (Fed. Cir. 2015) (distributed learning control module). Thus, whenever a claim element is named functionally, the prosecutor should make sure that specification broadly discloses corresponding structure, in case the courts decide to construe the element under 112(f).