The process of claim construction - determining the meaning of patent claims - is the most important part of patent litigation. There are a number of rules or canons that courts can use in applying the interpretive sources to reach an understanding of what patent claims mean. The canon that has arguably had the most significant impact on claim construction is the doctrine of claim differentiation. The claim differentiation doctrine in its broadest reading provides that no two claims in the same patent should be interpreted to cover the same thing.
As a general matter, applying the doctrine of claim differentiation results in broader constructions of patent claims, because it is most commonly used to prevent defendants from limiting a broad genus claim to the range of embodiments actually disclosed or more explicitly recited in other claims. Sometimes this is the right result, because defendants are improperly seeking to limit broader genus claims to the preferred embodiments disclosed in the specification. But at other times it leads to problematic results.
In this article, I conduct an empirical review of claim differentiation decisions in the Federal Circuit and in the district courts, and I suggest limiting principles that can be used to distinguish good uses of the doctrine from bad.