Sign Language is gestural communication used as an alternative or replacement for speech. Sign languages resemble oral languages in every way other than their modality. As with oral languages, sign languages are acquired spontaneously and have highly intricate, rule-governed grammar and phonology. The three classes of features that make up individual signs are hand configuration, movement, and position to the body. Sign languages include those of Trappist monks, who have a rule of silence, and Plains Indians, where speakers of mutually unintelligible languages communicated freely. Australian aborigines and people of the Sudan and the Sahara also have a complete sign language. Many languages have conventionalized body gestures elaborated to accompany or supplement speech, e.g., the Neapolitan gesture language.
The widely used manual language of the deaf, or language of signs, was first systematized in the 18th cent. by the French abbé Charles Michel de l'Épée. It was brought to the United States by T. H. Gallaudet. As with any sign language, only a small percentage of signs suggest the form of thought they represent. Such sign languages also may have a syntax and grammar that differs dramatically from the language spoken locally. This is true, for instance, of American Sign Language, which, developed for the deaf, is a non-English system used in the United States and parts of Canada. A number of written systems for representing manual languages have been developed, and dictionaries of signs have been compiled. Often sign language is taught along with speechreading (see lip reading) and with a manual alphabet, i.e., a method of forming the letters of the alphabet by fixed positions of the fingers in the air.