The shapes of forests of inheritance trees can affect the amount of code reuse in an object-oriented system. Designers can benefit from knowing how structuring decisions affect reuse, so that they can make more optimal decisions. We show that a set of objective measures can classify forests of inheritance trees into a set of five shape classes. These shape classes determine bounds on one reuse measure, the relative degree of code savings. An initial empirical study shows that the measures can be implemented and that inheritance forests can be objectively and automatically classified into one of the five shape classes.