Rule combination can contribute to morphological simplicity. Synchronically, rule combinations (like word combinations) are sometimes stored as formulaic units, and this fact contributes to a morphological system’s processing simplicity, since accessing a stored rule combination directly is simpler than decomposing that combination into its component rules for separate lookup. Stored, formulaic rule combinations may also contribute to diachronic simplifications of a language’s morphology, since they are the locus of reanalyses that may eventuate in “affix telescoping,” the development of a rule combination into a simple rule. But affix telescoping is not a monolithic phenomenon; it involves the reduction of a rule combination’s combinatory transparency along at least four dimensions. Thus, it is possible to find rule combinations that are progressing toward reanalysis as simple rules without yet having reached the point of reanalysis.

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.