Tunen tone changes and derived phonological contrast

Recent work in generative phonology has shown that the need to preserve underlying contrast, or to avoid surface homonymy among underlying distinct forms, may sometimes affect the application of rules. This can take the form of blocking rule application, or of ordering rules in such a way as to avoid merging of contrasts. Another possible way in which grammars might avoid the derivation of homonymous surface forms has remained unexplored, however: it is conceivable that a language might have special rules whose function is to differentiate underlyingly identical forms on the surface. It is shown here that, in the Bantu language Tunen, although many phonemically identical nouns are tonemically distinct underlyingly, a residue of pairs of nouns remains which are phonemically and tonemically identical in underlying form. These nouns do not remain homonymous in all surface environments, however, but are sometimes distinguished tonally through the operation of a special rule. Despite the surface tonal differences which sometimes appear, it is not possible to introduce underlying contrasts by taking the different tonal alloforms as the underlying representations in these cases. The Tunen situation seems in some sense very unusual, though in another sense it can be said to be expected. Certain theoretical conclusions are drawn at the end of the paper.