My opponent made a move but didn't press the clock. I didn't notice, so I made my move and then realised that the clock is already pressed. Am I allowed, under FIDE rules, to press the clock two times: first for my opponent and then for me?
According to the FIDE Laws of Chess:
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.
7.5.3 If the player presses the clock without making a move, it shall be considered and penalized as if an illegal move.
Together these mean that you would be wise to press your opponent's clock, press your clock and then tell your opponent that you have done so.
If you don't do so then 6.2.2 means that your opponent is entitled, after you move, to press the clock to complete their previous move. If you are not at the board then your clock can run while your opponent thinks.
7.5.3 means that you should tell your opponent so that they are aware and do not "remember" they forgot to press the clock and press it without making a move.
By my reading of the FIDE rules, I'd say no, you're not allowed to press your opponent's clock.
Article 6.2.1 makes it clear that pressing the clock is an integral part of the move, and the duty of the player who made the move:
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. This “completes” the move.
This is emphasised further in 6.2.2:
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.
By a strict reading of 6.2.1, you would not even be allowed to move your piece before the opponent has stopped their clock, so the question would not apply. However, 6.2.2 does allow for the possibility of making a move before the opponent has stopped their clock. A contradiction.
Here, it's useful to consider the most likely case where this conflict could happen: with the players executing a sequence of moves in quick succession, it could happen that one moves, and the other starts their move before the clock has been pressed. Even so, the players must press their clocks after each move.
The legality of this has been discussed in great detail at least around 2008, with varying interpretations, see e.g. these posts on the US Chess Federation website: Response to Irina's Open Letter and Move Theory.
Accepting that you were not initially at fault for moving before the opponent pressed their clock, note that the phrasing of those articles is normative: "a player shall stop their clock", and "a player must be allowed to stop their clock". By the rules, as they are written, there is no other option, and therefore by pressing the opponent's clock on their behalf, you would run directly afoul of 6.2.2 as that would disallow the opponent from doing it.
Apart from that, reaching to their side to push their clock could well be unusual enough to be seen as distracting. (In the worst case, both would awkwardly try to press the clock on the same side at the same time.) Distracting the opponent is forbidden:
11.5 It is forbidden to distract or annoy the opponent in any manner whatsoever. This includes unreasonable claims, unreasonable offers of a draw or the introduction of a source of noise into the playing area.
Going further than the FIDE rules, the USCF Blitz rules explicitly list what a player is allowed to do with the clock, and pressing the opponent's side is not listed, other than to fix a clock not ticking.
So, the options you are left with, appear to be either
- to gently remind the opponent to press their clock in an undistracting manner (if you're feeling sportsmanlike); or
- to let them run out of time (if not).
The consensus at least here on chess.SE appears to be that the former is acceptable (see e.g. Should you let your opponent know if they have forgotten to press their clock?). If it does not appear to be doable without distraction, option 2 is still left.
To add to the contradiction mentioned above, 6.2.1 continues with
... A move is also completed if:
22.214.171.124 the player has made his next move, when his previous move was not completed.
This could happen when both players move (presumably in quick succession), with neither pressing the clock and the player now in turn makes their next move and finally presses the clock. (Under a time control with per-move increments, or with a clock counting the moves, this could be problematic.)
Other than this and the articles quoted above, I can't see the rules exactly say anything about the case where both players move and the first leaves the clock unpressed for an extended time. Partly the rules seem to be written with the assumption that this shouldn't happen, but also they appear to say that the player must still be allowed to complete their move by pressing the clock (or by making another move and then pressing the clock) — even if this happens after the other player has left for a bathroom break.