M-9 was the designation of a sign language using two hands.
In 2365, this language had been recorded in the linguistic database of the United Federation of Planets. It was identified and learned by Lieutenant Commander Data in his attempt to communicate with Riva, a deaf mediator from Ramatis III. M-9 was one of five languages he chose to learn to identify the particular language Riva was using. As such, Data viewed multiple M-9 gestures on a screen.