• Estoppel, or more properly equitable estoppel, is closely related to laches. Whereas laches involves a mere failure to act, equitable estoppel requires some affirmative act by the patentee that leads an accused infringer to believe that it will remain unmolested (barking, but then failing to bite).

• The chief limitation on the interpretation or construction of words or phrases in a patent claim is file wrapper estoppel, a doctrine created by the courts that prevents a patentee from recapturing, through a broad interpretation of words or phrases in a claim, that which was previously surrendered in securing allowance of the claim.

• Thus, the scope of a patent claim is largely determined by the prosecution history of the patent (i.e., the file wrapper).

• Although mere silence does not give rise to equitable estoppel, extended inaction after issuing a notice of infringement may well do so.

• Equitable estoppel not only bars recovery for past acts of infringement, it renders the patent completely unenforceable with respect to that defendant. Never accuse others of infringement, nor threaten suit, unless prepared to take legal action.