1. If I touch a piece with my forearm by accident, can my opponent call touch-move?
  2. If the answer is "no", what if after my opponent calls touch-move, I believe him and move the touch piece before calling the arbiter - can I take it back?

According to the 2018 FIDE Laws of Chess (emphasis mine):

4.2.1 Only the player having the move may adjust one or more pieces on their squares, provided that he first expresses his intention (for example by saying “j’adoube” or “I adjust”).

4.2.2 Any other physical contact with a piece, except for clearly accidental contact, shall be considered to be intent.

4.3 Except as provided in Article 4.2, if the player having the move touches on the chessboard, with the intention of moving or capturing:
4.3.1 one or more of his own pieces, he must move the first piece touched that can be moved

Touching with a forearm seems to be "clearly accidental contact", so it shouldn't trigger the touch move rules.

In the scenario where you move the piece because your opponent said you must, and then want to take it back, it's less clear. You generally cannot take back moves (and, if you've moved, then obviously you have now touched the piece with the intent of moving it.) However, your opponent has acted inappropriately; at the very least they've distracted you during your move. It's possible the arbiter would give some sort of penalty for this, especially if your opponent knew touch-move didn't really apply. You should summon the arbiter as soon as possible to get a ruling.

  • 4
    Answer is correct - its definitely not a touch move, but I would add that any tournament higher than club internal you would definitely not be allowed tacking back your move - as it was explained - you should believe arbiter not opponent, in kids tournament there are popular cases when one says checkmate when its actually not, but if opponent believes, accepts it - the game is lost; same here - reasons does not really matter, if you moved, if you resigned - it's done.
    – Drako
    Oct 12 '18 at 6:32
  • In your local club were just friends playing - arbiter(senior club member) could explain and allow taking back if opponent would agree
    – Drako
    Oct 12 '18 at 6:39
  • 2
    Absolutely this. However, in tournament play, unless it's aimed at amateurs, players should not be clumsy with their bodies any more than they should be clumsy with their minds.
    – corsiKa
    Oct 12 '18 at 19:43
  • @corsiKa So, in your world, anybody with any kind of physical disability or co-ordination issue should be banned from playing OTB chess? Wow. Oh, wait, just banned from playing professional chess. That's so much better. Oct 13 '18 at 19:09
  • 1
    "4.9 If a player is unable to move the pieces, an assistant, who shall be acceptable to the arbiter, may be provided by the player to perform this operation." - FIDE Laws of Chess.
    – D M
    Oct 15 '18 at 21:06

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