You are playing western Chess or Shogi (don't think this is ever an issue in Xiangqi)and your opponent has left his piece on the line between two squares and then pressed the clock. Maybe he does this repeatedly.
\* Is this specifically covered in the rules?
\* Do you have to clarify what the move is on your own time?
\* Is what the opponent doing actually illegal?
\* Say you do not challenge the move, perhaps assuming its the more logical f4 rather than the weird f3, or maybe the piece is 75% on f4 and 25% on f3. Maybe its a time scramble and he is not notating. Now you play your queen to e3 whereupon your opponent takes it! Can you challenge the move to the arbiter?
This situation did actually happen to me. Fortunately, after a few such moves like this with me clarifying the move each time, I asked the arbiter to observe the game when the moves miraculously ended up in the exact centre of the target square making me look more than a little foolish!
I have looked through the FIDE rules to see anything pertaining to this, and here are the two rules that should be applicable to this situation:
7.3 If a player displaces one or more pieces, he shall re-establish the correct position on his own time. If necessary, either the player or his opponent shall stop the clocks and ask for the arbiter’s assistance. The arbiter may penalise the player who displaced the pieces.
7.5 If during a game it is found that pieces have been displaced from their squares, the position before the irregularity shall be re-instated. If the position immediately before the irregularity cannot be determined, the game shall continue from the last identifiable position prior to the irregularity. The clocks shall be adjusted according to Article 6.13. [Arbiter's best judgement] The game shall then continue from this re-instated position.
As i recall from the Chess Federation of Canada handbook, you can restart your opponent's clock without making a move, and demand that the piece be placed on the square properly. If there is any fuss, or if it happens again, then call the arbiter. It happens fairly frequently when a player knocks over other pieces in a time scramble.
I don't have the CFC handbook handy to find the exact rule, but FIDE 7.3 seems to cover the situation.
You can probably restart the clock, but it might actually be you who is in the wrong if you are seen as overly fussy. When a piece is knocked over you are clearly entitled to restart the clock. If the piece is just a millimetre over the edge you might want to get the arbiter if they are continually sloppy. Where there is clear ambiguity I would happily restart the clock, if it isn't clear what the move is and the opponent is continually sloppy I'd ask the arbiter to observe.
BTW do not adjust the opponents piece you are supposed ask your opponent to adjust there own piece!
Don't feel foolish for informing the arbiter, often the fact an arbiter is around means a player is more focused on being as well behaved as possible.
Actually I see no problem in centering an opponent piece if slightly off but unambiguous positioned.
There might be a problem in one of the following, though: obsessionally and constantly centering opponent's pieces; adjusting an opponent piece which is in an ambiguous position (and therefore making a call on its actual position); changing the orientation of opponents non symmetrical pieces (mostly an issue with knights but for a very sensitive player bishops be as well)...
BTW, in Brazil we have a funny expression for (intentionally) the act of playing a piece between two squares. It's called " jogar no semi-pleno". That means to play at semi-pleno, the last being the Portuguese term for a " Split bet in a roulette game.
I don't see a problem with centering an opponent's piece but I had someone complain, even though I did it in my own time and I'm fairly sure it was a pawn!
1. Don't play roulette (unless you can sneak a computers in with accomplices or find that the roulette wheel has a flaw that allows an edge to you).
2. Don't touch someone elses chips it is bad etiquette and you may get mistaken for a thief!
Some clever engineers measured biases in roulette wheels, and exploited them with the use of computerized timing counters in the their shoes. And this was in the 60s!
It may not be the rule, I took an arbiters word about the not adjusting an opponent's piece, after I was surprised to get a complaint, when I was convinced I'd done nothing wrong (done in own clock time and no reorienting knights).
Darse is right I was referring to the computerized timing of a roulette wheel.
To clarify, in case you don't want to read the book:
A motorized casino roulette wheel spins at a steady rate, and the croupier launches the ball in the opposite direction. The player would click the counter in his shoe every time the ball passed a given reference point. Based on the calculation of its speed, they could estimate which quadrant of the wheel the ball was more likely to land in. They had betting strategies to cover that quadrant, and they would be correct often enough to have a positive expected value overall.
Obviously way off topic, but i'll allow it because it's interesting. :-)
The British version of the book is "The Newtonian Casino", I enjoyed it very much.
The main bias of the roulette they exploited was the tilt. Even a tenth of a degree allowed winning odds. Nowadays it is hard to find tilted roulettes. See http://cryptome.org/roulette-problem1.pdf.
Little Golem