In a reply to another question, I made an explicit presumption that under FIDE Laws, a player can move as soon as the opponent’s move has been made. But someone wondered if this was this really correct.
The Laws make a distinction between a move being ‘made’ and a move being ‘completed’ (when the clock has been hit). As far as I can tell, the former (under which the waiting player is said to ‘have the move’) is sufficient for allowing the next player to move. But there is no rule which says you can move when you have the move, maybe because it’s deemed too obvious.
As far as I can see the following scenarios are legitimate:
A: move B: move B: hit clock but nothing happens A: hit clock B: hit clock A: move B: move A: move A: hit clock B: move B: hit clock
And might as well list a related scenario:
A: move B: wait for A to realise A: hit clock
But I am not an arbiter. Can a player move before their opponent hits the clock?