The most familiar sign language systems today are those developed for the deaf and hearing-impaired. However, non-verbal communication through bodily movements is much older than these relatively recent systems. The need for secrecy, vows of silence, and language barriers between people of different cultures have spurred people to invent non-verbal language systems. Members of religious orders sworn to silence often rely on simple gestures rather than a coded system to communicate with one another; the English historian and cleric Venerable Bede (673-735) devised a system in which manually signed numbers, representing letters, were used to spell out words. Chinese and Japanese, whose languages use the same written characters but pronounce them differently, will sometimes trace characters onto another's palms to communicate. In the nineteenth century, North American Plains Indians developed a system of signs bridging tribal language barriers. The system did not match words directly with...