The study of phonological structure and patterns across languages is seen by contemporary phonologists as a way of gaining insight into language as a cognitive system. Traditionally, phonologists have focused on spoken languages. More recently, we have observed a growing interest in the grammatical system underlying signed languages of the deaf. This development in the field of phonology provides a natural laboratory for investigating language universals. As grammatical systems, in part, reflect the modality in which they are expressed, the comparison of spoken and signed languages permits us to separate those aspects of grammar which are modality-dependent from those which are shared by all human languages. On the other hand, modality-dependent characteristics must also be accounted for by a comprehensive theory of language. Comparing languages in two modalities is therefore of theoretical importance for both reasons: establishing modality-independent linguistic universals, and accounting for modality-dependent structure and organisation.
