How did a French teacher help create American Sign Language?

Laurent Clerc, a deaf teacher trained in Paris, brought French Sign Language to the United States, and with it the seed of American Sign Language. But ASL was not simply French Sign Language carried across the sea. It grew at the school he helped open in 1817, where the children arrived with signs of their own, and the mixing produced a new language.

The story starts in Paris, where Clerc studied and then taught at the national school for deaf pupils, learning from Abbé Sicard and from Jean Massieu, who was himself deaf. In 1815 he travelled with them to Britain and met Thomas Hopkins Gallaudet, an American looking for ways to teach deaf children. A year later Gallaudet invited Clerc to come to the United States. On the voyage, Clerc learned English from Gallaudet and Gallaudet learned French Sign Language from Clerc.

In 1817 they founded the first school for the deaf in North America, in Hartford, Connecticut, now the American School for the Deaf. Clerc taught in French Sign Language. But the pupils brought village sign languages, home signs, and the sign language of Martha's Vineyard, which was the home of the largest group of students in the early decades. Out of that contact emerged ASL.

The numbers show a relative, not a copy: about 58% of signs in modern ASL are cognates of Old French Sign Language signs. That is far less than the 80% usually needed to call two languages dialects. Clerc himself noted that he often learned signs from his pupils rather than teaching his own.