The sign language is very important thing because this form of language every body understand.
In addition to sign languages, various signed codes of spoken languages have been developed.
For example, a teacher and deaf student of English in the United States might use Signed English to cite examples of English usage, but the discussion of those examples would be in American Sign Language
Sign languages exploit the unique features of the visual medium.
In much the same way that geographical or cultural forces will isolate populations and lead to the generation of different and distinct spoken languages, the same forces operate on signed languages and so they tend to maintain their identities through time in roughly the same areas of influence as the local spoken languages.