The sign for "square" can be used to indicate nearly anything
with a square perimeter such as a billboard. This is a very "general"
sign good for referring to whatever
is "square shaped."
Draw a square in the air with your index fingers.
SQUARE / BOARD / SIGN / BILLBOARD:
Add black to the sign for "square" and you'll have a "blackboard."
BLACK + BOARD = BLACKBOARD:
"Do you think the teacher should write on the board more often?"
= SQUARE-[board], YOU THINK HE/SHE TEACHER SHOULD WRITE MORE?
If you add "WHITE" to the sign "BOARD" you'll have "whiteboard."
If you add "GREEN" to the sign "BOARD" you'll have "greenboard" (heh) okay
I'm just yanking your chain ... actually it will mean "green screen"
like they use in Hollywood for special effects in movies.