His head moves up and down.
His head moves down and up.
Do we have to use #1? What about #2? Can we use both?
His head moves up and down.
His head moves down and up.
Do we have to use #1? What about #2? Can we use both?
"Up and down" is a fixed phrase meaning "in one direction and then in the opposite direction, especially repeatedly". The two opposite directions are often vertical, but not exclusively; I can walk up and down a level path. To reverse it when describing that kind of action would be non-standard and viewed as an error by most native speakers. If only two vertical motions altogether are being described, and the first is downwards, then we could say, for example: "he moved his head down and up again".