Patent claim is measured by the words used in its claims, and therefore, any clause used is critical. A patent attorney usually plays with the language of a claim to reach a desired scope of protection. A powerful tool available to the patent attorney to describe their invention in the claims of a patent is the ability to utilize functional language-to describe what an invention does as opposed to what it is.
For example, rather than claiming a pen, the patent attorney may claim a means for writing. The words “means for writing” describe a much larger class of items than the word “pen,” such as pencil and the like. Such clause in a patent claim is known as “means-plus-function clause.”
A means-plus-function clause (also sometimes referred to as “means clause”) is useful in various strategies for drafting proper claim set, such as in:
- The means-plus-function clause can be used to describe the function of a particular element. The means clause is used especially where the function or action, rather than the specific structure that causes or accompanies the action, is important to the claim.
- Further, “a means for performing a function clause” is useful where there are actually two separate functions being performed and the two functions accomplish a further broader objective, so that the “means for performing the further function” encompasses both of the more specific functions.
The Patent Office usually gives a “means” limitation the broadest reasonable interpretation. However, the said “means” as stated in the claims shall be properly defined in the accompanying specification of the patent document. For example, the means-plus-function claims were very popular until the early 1990s. The idea that a smart patent writer could claim an apparatus without expressly limiting the elements by a particular structure is very enticing. Thus, a pen could be broadly claimed as a “means for writing”, and a container could be broadly claimed as a “means for carrying a substance”. The problem, of course, is that the essence of a structure claim (apparatus/device/composition/ software) is that it defines structure, and means-plus-function claims circumvent structural limitations precisely because they define elements by their function.
Usually, these days it is recommended to avoid means-plus-function claims. The scope of equivalents results in that a means-plus-function element is narrower than the scope afforded other elements under the doctrine of equivalents, and it is usually trivial to revise a draft claim to eliminate the means-plus-function language. The means-plus-function claims comes with a risk that failing to clearly link the structure disclosed in the Specification with the function recited in the claim can invalidate the claim for failure to fulfil the requirement of the written description
However still some attorneys widely use this practice. If there is a rejection by an examiner, some explanation may be provided as to the “means” clause. Before deciding to argue patentability based on a means-plus-function limitation in response to an examiner’s rejection, it must be considered that including a means-plus-function clause may create a prosecution history estoppel which limits the patentee’s protection.