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?
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1If someone dips their head then it goes down and up. – Weather Vane Apr 11 at 19:18
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You can use either one, but only #1 would sound idiomatic. – J.R.♦ Apr 11 at 20:22
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An alternative to #2 (US) could be, "He nodded once." – mkennedy Apr 11 at 20:41
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"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".
answered Apr 11 at 19:33
