Can a player move before his opponent hits the clock? 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?
There is no requirement to wait until the opponent has pressed the clock before making your move. However issues can arise with getting the increment to which the player is entitled for every move made if the clock is not pressed and this is spelled out in the FIDE Laws of Chess:


  6.2.1 During the game each player, having made his move on the chessboard, shall stop his own clock and start his opponent’s clock
  (that is to say, he shall press his clock). This “completes” the move.
  A move is also completed if:
  
  6.2.1.1   the move ends the game (see Articles 5.1.1, 5.2.1, 5.2.2, 9.6.1 and 9.6.2), or
  
  6.2.1.2 the player has made his next move, when his previous move was not completed.
  
  6.2.2 A player must be allowed to stop his clock after making his move, even after the opponent has made his next move. The time between
  making the move on the chessboard and pressing the clock is regarded
  as part of the time allotted to the player.


From 6.2.1.2 it is clear that is fine for player B to make his move before player A has pressed the clock to complete his previous move.

From 6.2.2 it is clear that in that case player A is entitled to press the clock for that move even after player B has made a quick move.

The act of pressing the clock fulfils two functions:


It "completes" the move. This is important because an illegal move is
not punishable and can be retracted without penalty if the move
hasn't been completed.
It is the action which gives the player the increment to which he is entitled.