Wikipedia
Letter board
A letter board may refer to two devices.
Usage examples of "letter board".
He asked for the written copy, which he found stuck into the letter board beside the main doors.
Rainey had briefly made do with a letter board, using her right hand, in which she still had good movement, to pick out the words she was trying to say.
I took her over to the big letter board so she could spell out words and answer my questions.