There are two general categories of language involving the use of signs: alternate sign languages and primary sign languages. By definition, an alternate sign language is a system of hand signals developed by speakers for limited communication in a specific context where speech cannot be used. In some religious orders where there are rules of silence, restricted alternate sign languages are used (e.g. by monks in a monastery). Among some Australian Aboriginal groups, there are periods (e.g. times of bereavement) when speech is avoided completely and quite elaborate alternate sign languages are used instead. Less elaborate versions are to be found in some special working circumstances (e.g. among bookmakers at British racecourses or traders in commodity exchanges). In all these examples, the users of alternate sign languages have another first language that they can speak.

In contrast, a primary sign language is the first language of a group of people who do not use a spoken language with each other. British Sign Language (BSL) and French Sign Language (SLF), as used for everyday communication among members of the deaf communities of Britain and France, are primary sign languages. Contrary to popular belief, these different primary sign languages do not share identical signs and are not mutually intelligible. British Sign Language is also very different from American Sign Language (ASL) which, for historical reasons, has more in common with French Sign Language.

We will focus our attention on ASL in order to describe some features of a primary sign language, but first, we have to account for the fact that, until fairly recently, it was not treated as a possible language at all.