Most people, when they think about it, assume sign languages are visual translations of spoken languages — that British Sign Language is a hand-based version of English, and American Sign Language is roughly the same. They aren't. Sign languages are independent natural languages with their own grammar, vocabulary, and history.

In 1815, an American minister named Thomas Hopkins Gallaudet traveled to Europe to learn how to teach deaf children. He went first to England, where the dominant educational tradition forced deaf students to lip-read and speak — sign language was actively suppressed. He found this approach hopeless. He continued to France, where the Royal Institute for the Deaf had been using a sophisticated sign language since 1760, refined by deaf educators themselves.

Gallaudet returned to the United States in 1817 with a French deaf teacher named Laurent Clerc. The two of them founded the American School for the Deaf in Hartford, Connecticut, teaching with French Sign Language. Over generations, it merged with home signs developed by American deaf communities and evolved into modern American Sign Language.

This means ASL and French Sign Language are linguistic siblings. ASL and British Sign Language — despite the shared spoken language — are completely unrelated. A British and American deaf speaker meeting for the first time will not be able to understand each other beyond a few cognates.

Sign languages have full grammar, including spatial syntax that doesn't exist in any spoken language. A signer can place an absent person in a virtual location and refer back to them by pointing. They can express simultaneous events in ways spoken language cannot. They evolve independently of the surrounding spoken language.

ASL is a French descendant. Hearing English speakers were not consulted.