What does the study of signed languages tell us about 'language'?

Linguists focusing on what all languages have in common seek to identify universals, tendencies, and other patterns to construct a general model of human language, Universal Grammar (UG). The design features of this model are that it must account for linguistic universals, account for linguistic diversity, and account for language learnability. Sign languages contribute to the construction of this model by providing a new source of data, permitting the claims and assumptions of UG to be rigorously tested and modified. One result of this research has been that the notion of 'language' itself has been clarified, clearly separating it from speech. It has also been possible to identify the design features of 'natural languages' themselves, and then to explain why pedagogical signing systems are not natural languages. This paper provides an overview of these issues.