ASL is an acronym for American Sign Language and it's basically a naturally complete language that is used as a form of sign-language for the deaf and du-mb communities in the United States of America. The American Sign Language is visual and gestural in nature, so it is expressed mainly by using signs such as movement of the hands, fingers and face (facial expression).
This ultimately implies that, like any conversation between speaking friends, a conversation in American Sign Language (ASL) relies on reading facial features.
