In this paper we develop a complete classification scheme for planar, spherical and spatial four-bar linkages. The goal of this classification is to note all of the subtleties of motion that an arbitrary set of four link lengths can define. A classification should exist only between the mechanisms that exhibit similar ranges of motion at both the input and output. In the planar case, three parameters being identified as either positive, negative, or zero are necessary to completely characterize all possible ranges of motion. In the spherical and spatial cases, four parameters being identified as positive, negative or zero are needed. The result is 27 classifications of planar mechanisms and 81 for spherical and spatial RCCC mechanisms.