This document is intended as a supplement to the official American Go Association (AGA) Rule Set http://www.usgo.org/resources/downloads/completerules.pdf. Its purpose is to clarify ambiguities and provide guidance to referees and tournament directors. Any remaining ambiguities not covered here should be resolved by reference to the "spirit" of the rules, to common sense, or to the referee where necessary.
Our definitive version of these rules are The BGA Rules of Play (PDF).
The theory behind of these rules is discussed in an appendix: The Equivalence Theorem (PDF).
The phrase "board-play" means placing a stone of the colour belonging to the player on an unoccupied intersection. The word "play" means placing a stone on an unoccupied intersection and removing any opponent's strings that are consequently captured (if any). The word "move" means a play or a pass.
The specified komi (7½ to white in an even game and ½ in a handicap game) applies only to play on a 19x19 board.
On boards smaller than 19x19, a non-integral komi should be used since avoidance of jigo is clearly part of the spirit of AGA rules.
Tournament organizers are free to specify deviations from the rules if they wish but these must be made explicit in the tournament publicity. Common rule departures would be board size and where to place handicap stones as well as different komi. Use of an integral komi (allowing jigo) and the use of boards other than 19x19 should be regarded as deviations and publicised accordingly, even though it is not explicitly forbidden by AGA rules.
We use the word "ko" in the traditional way to mean a repetitive cycle of length 2 and "superko" for anything more complex.
Rule 6 extends the normal meaning of the word "ko" to include what we normally call "superko".
We clarify superko to mean that a player should not make a play which re-creates a position previously created by a play of the same player.
For purposes of Rule 8, there are just two types of illegal move:
Any other plays which unintentionally violate the rules are irregularities and are discussed in the Interpretation 8.3 below.
For type (i), the "illegal move" consists of the whole sequence of two or more moves played consecutively by the offending player.
If the illegal move is noticed before the next move is played, AGA Rule 8 about substituting a pass applies.
If one or more further moves are played before an illegal move is noticed, then for both types of violation, the players may agree to rewind to the position preceding the illegal move as in AGA Rule 8. We interpret AGA to imply that this is done without penalty.
In the case of a type (i) violation, if the players do not agree to rewind, the referee must be called to resolve the situation.
In the case of type (ii), for self-capture, the relevant stones should simply be removed as prisoners. For ko and superko continue with the next move, perhaps recording subsequent moves to identify the repeating cycle.
There are several other forms of stone placement which violate the rules. These are irregularities, rather than illegal moves.
It is assumed that irregularities happen unintentionally. By default, as soon as it is noticed by either player, the board should be corrected without penalty to the mutual satisfaction of both players. The following are the known irregularities, with common methods of fixing them.
(i) Failing to remove all the captured stones. Remove the stones and give them to the opponent as prisoner.
(ii) Removing stones that still have one or more liberties. Replace all the removed stones. The original 'capturing' stone stays on the board.
(iii) Placing handicap stones in the wrong position. If this is noticed immediately before White's first move then set the stones correctly. Otherwise just carry on.
(iv) Placing a stone on an occupied intersection. If noticed immediately, remove the stone from the board and it is the opponent's turn to play. If noticed later just remove the stone from the board and return it to the owner's bowl.
(v) Placing a stone ambiguously in between intersections. Ask the owner of the stone to position it on its intended intersection.
(vi) Playing a stone of the wrong colour. If noticed immediately, replace the stone with one of the right colour and complete any consequent captures. If noticed later, try and fix the board position as best as possible by agreement of both players. A stone count can be used to decide if a stone of the wrong colour has been played.
The AGA Rules specifically give playing on an occupied intersection as an example of an illegal move. We regard this as an irregularity, as discussed above.
Rule 10 specifies that, if both players pass twice in succession (four consecutive passes), game end is triggered with all stones remaining on the board. This is intended to cover only the abnormal case where the players disagree about the status of some group(s) but both refuse to continue playing to physically capture stones asserted to be dead. Normally, multiple resumptions of competitive play may occur until all disputes are resolved.
Every occurrence of two consecutive passes indicates a belief by both players that the game is over. They should then try to reach agreement as to which groups are dead. This can happen indefinitely many times. Only if they fail to reach agreement and also refuse to play on (by both passing a second time making four consecutive passes in total) does the game end with all stones remaining on the board.
There is no onus on either player to justify his or her opinion about the status of any stones before a resumption occurs. If necessary, any player claiming that particular stones are dead must capture them during resumed play.
Ron Bell, Geoff Kaniuk