A number of scholars have argued for the need to postulate principles of rule combination in morphological theory; according to such principles, two rules may combine to produce a more complex rule. Several kinds of evidence motivate the postulation of such principles, which afford new and revelatory explanations for a range of familiar morphological phenomena. Central to these explanations is a set of four characteristics (component independence, phonological transparency, semantic transparency, and domain subsectiveness) that combined rules possess by default but from which they may also deviate. This set of characteristics has both synchronic and diachronic significance. Synchronically, they elucidate the nature of potentiation, the relation between two affixes A and B such that stems created by means of A extend the domain of stems to which B may subsequently attach (Aronoff , Williams ). Diachronically, they illuminate the nature of affix telescoping, the diachronic correspondence of a sequence of two affixes at one stage in a language’s history to a single affix at a later stage (Booij , Haspelmath ). The evidence discussed here lends additional strength to the conclusion that principles of rule combination are a necessary addition to morphological theory.