Linguistics on Jeopardy - $400

In honor of the special Jeopardy tournament this week, I have decided to do a daily post with past Jeopardy clues related to language and linguistics.

The category: Linguistics

The $400 clue: These systems of communication that use the hands can be as rich & complex as spoken tongues.

Answers and episode credits will be posted at the end of the week. There will be a special prize for the first ten people that submit the correct answer (question) each day all week.