Sign languages (also known as signed languages) are languages that use the visual-manual modality to convey meaning. Sign languages are expressed through manual articulations in combination with non-manual elements. They are most commonly associated with deaf people. Sign languages are full-fledged natural languages with their own grammar and lexicon. At least 137 sign languages are recognized world-wide. Many are not mutually intelligible with each other, although there are some nearly universal commonalities.

Captive great apes, such as chimpanzees and gorillas, have been taught by humans to communicate with humans and with each other using sign language, physical tokens, lexigrams, and mimicking human speech. Some primatologists argue that these primates' use of the communication tools indicates their ability to use "language", although this is not consistent with some definitions of that term.

Sign language in A Different Flesh[]

The sims of the Americas had a capacity to understand human languages, but were unable to reproduce the speech sounds. With the help of humans, certain sim communities developed sign languages which became reasonably consistent and well known enough to become the lingua franca of large regions, both among sims themselves and used by sims for the benefit of their human neighbors. Despite this, sims never learned to read printed alphabets, nor even to interpret geographic marks on a map, although such education was repeatedly attempted beginning in the 16th century.