People who write patents walk a fine line.  A broad claim increases the stuff that infringes, but it also raises the risk that the patent fails some test for validity.  A Federal Circuit ruling today pointed up one of those invalidity rules.

A common way to claim broadly involves using "means-plus-function" language.  The claim says something like "means for producing eternal movement".  Or it speaks of a "means capable of turning lead to gold".

The careful reader will note that in neither case does the claim tell you how, respectively, to build a perpetual motion machine or to practice alchemy.

The patentistas will tell you that, before you get to the claims, you must disclose a "structure", usually in the specification of the patent.  And you must do it in terms that someone who knows the field fairly well can figure out.

The Federal Circuit re-affirmed the means-plus-function rule today in Blackboard, Inc. v. Desire2Learn Inc., No. 08-1368 (Fed. Cir. July 27, 2009).  It held that the district court erred by not deeming claims 1-35 in a patent invalid as not definite enough.