Various approaches have been proposed to provide communication between CAD systems and process planning systems, including automated feature extraction, design by features, and human- supervised feature extraction. Regardless of which approach is used, a major problem is that due to geometric interactions among features, there may be several equally valid sets of manufacturable features describing the same part - and different sets of features may differ in their manufacturability. Thus, to produce a good process plan - or, in some cases, even to produce a process plan at all - it may be necessary to interpret the part as a different set of features than the one initially obtained from the CAD model. This paper proposes a way to address this problem, based on an algebra of features. Given a set of features describing a machinable part, other equally valid interpretations of the part can be produced by performing operations in the algebra. This will enable automated process planning systems (such as [Nau87]) to examine these interpretations in order to see which one is most appropriate for use in manufacturing.