British Sign Language

British Sign Language was a type of communication used between deaf people in the United Kingdom.

In 2119, Cass, the second in command for an underwater mining facility in Scotland, communicated with her team only in British Sign Language, which was translated by fellow employee Tim Lunn. The Twelfth Doctor attempted to respond to her in BSL but soon realised he had forgotten how to, apparently having deleted it in favour of semaphore. (TV: Under the Lake)

Behind the scenes
British Sign Language was used on-screen by a character for the first time by Sophie Stone, who played Cass in Under the Lake.