Languages Generated by Two-Level Morphological Rules

The two-level model .of morphology and phonology arose from work on finite-state machine descriptions of phonological phenomena. However, the two-level rule notation can be given a precise declarative semantics in terms of the segmentation of sequences of pairs of symbols, quite independently of any computational representation as sets of finite-state transducers. Thus defined, the two-level model can be shown to be less powerful, in terms of weak generative capacity, than parallel intersections of arbitrary finite-state transducers without empty transitions (the usual computational representation). However, if a special boundary symbol is permitted, the full family of regular languages can be generated. Two-level morphological grammars may, without loss of generality, be written in a simplified normal form. The set of two-level generated languages can be shown to be closed under intersection, but not under union or complementation.