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PAR (principal axis representation) for rotational parts (i.e., solids of revolution) is proposed as an internal representation scheme for constructive solid geometry (CSG). They key idea of PAR is to represent an object uniquely by its principal axis and a set of boundary curves. Since PAR is in an evaluated form, geometrical properties of parts can be computed more directly and efficiently from this evaluated representation than from the original CSG tree. The scheme is described, and operations (e.g. union) on PARs are defined. The uniqueness of the scheme is proved. An algorithm that converts a CSG representation into PAR is presented along with examples. The equivalence of PAR to the CSG scheme for rotational parts is proved.