The judicial doctrine of claim differentiation permits courts to presume that claims cover different inventions. This provides the basis for the general rule of not reading limitations of a dependent claim into a parent claim. Otherwise, the parent claim and the dependent claim would cover the same invention. For example, it is common practice to have a claim broadly define an element of an invention. Having a dependent claim recite details of that element is also common. Under the doctrine of claim differentiation interpreting the parent claim to include the details recited by the dependent claim would be improper.
However, if the parent claim defines the element using means plus function language, the parent and dependent claim may be interpreted to define the same invention. The reason for this is that the Federal Circuit, in Laitram Corp. v. Rexnord, Inc., 939 F.2d 1533, 1538 (Fed. Cir. 1991), held that the judicial doctrine of "claim differentiation" does not override the statutory requirements of 35 U.S.C. § 112 (sixth paragraph). Under this provision, an element of a claim expressed in means plus function language "shall be construed to cover the corresponding structure, materials, or acts described in the specification and equivalents thereof."
In Laitram the claim at issue defined a "means for joining." A dependent claim, which was not in suit, defined a structure of the "means for joining" disclosed in the specification. The alleged infringer argued that its product did not include the structure shown in the specification, and thus it did not infringe the claim. The patent owner countered that the doctrine of claim differentiation prevented the structure recited by the dependent claim from being read into the broad parent claim. Thus, according to the patent owner, without the limiting details recited by the dependent claim, the broad claim was infringed.
The Federal Circuit first held that the judicial doctrine of claim "differentiation is a guide, not a rigid rule. If a claim will bear only one interpretation, similarity will have to be tolerated." Laitram, at 1538. The Court went on to explain that
A means plus function limitation is not made open ended by the presence of another claim specifically claiming the disclosed structure which underlies the means clause or an equivalent of that structure. Laitram, at 1538.
The underlying principle is that interpreting a means plus function element to cover a specific structure results from reference to the specification as required by 35 U.S.C. § 112 (sixth paragraph). It does not result from reference to a dependent claim. As the Court in Laitram correctly noted, a judicial doctrine cannot override the statutorily required claim interpretation.