The AGA rules are the rules of Go adopted by the American Go Association.
The rules are intentionally formulated so that there is almost no difference whether area scoring or territory scoring is used. This is made possible by requiring white to make the last move and incorporating "pass stones". This means that if white passes first, he or she must pass again after black, handing over a second pass stone. Eyes in seki situations are counted as territory in territory scoring and are part of the area in area scoring.
They prohibit suicide.
(They are therefore quite similar to Chinese rules.)
They use the situational superko rule:
- It is illegal to play in such a way as to repeat a previous board position from the game, with the same player to play.
In addition white must make the last play.
In theory the rules allow free placement of handicap stones, but in practice the traditional Japanese placement is usually used. This is mentioned in the commentary at http://www-2.cs.cmu.edu/~wjh/go/rules/AGA.commentary.html
The complete text of the rules can be found here: http://www.usgo.org/resources/downloads/completerules.pdf
Notice that the 1991 date in the above document is wrong: the rules were changed in August 2004, with komi set to 7.5 rather than 5.5, effective 2005; see point 9 in http://www.usgo.org/resources/downloads/2004-minutes.pdf
 Territory counting (with pass stones) is used by default unless the players agree before starting play to another counting method. The Mathematics of Scoring shows the equivalence algebraically at the end of the page.
Deacon John The Mathematics of Scoring shows that territory counting (with pass stones) is mathematically equivalent to area counting. Area scoring (with pass stones) can not be mathematically equivalent to territory scoring, because area scoring and territory scoring do not always give the same result, even in commonly occurring situations. The Gun Eight seki pattern provides a nice example of the difference between area scoring and territory scoring in a commonly occurring situation.
 Robert Jasiek: Many readers of the rules text read it as if it were meant to describe situational superko (but the major author of the rules, Terry Benson?, insists that natural situational superko was meant).
According to http://www.britgo.org/rules/aga.html the BGA has decided to adopt the AGA rules in october 2007. The document mentions that the board is proposing to get this change accepted (or denied) at the 2008 Annual General Meeting. Does anyone know if the 2008 AGM has already happened, and what the result was if it did? --Herman Hiddema
RobertJasiek: The AGM has taken place during the 2008 British Go Congress. The motion to adopt the AGA Rules has been accepted unanimously.
Herman Hiddema: Ok, thanks! I couldn't find anything about the AGM on the BGA website :-)
Strongeye: The AGA rules specify territory as: "Those empty points on the board which are entirely surrounded by live stones of a single color are considered the territory of the player of that color." But neither the rules, nor the commentary text, specify if seki positions are a) alive or dead, or b) eyes in seki strings count as territory. A direct and literal interpretation as far as I can tell would mean that they do, but it isn't explicit in this. Can anyone clarify? Thanks.
- Strongeye: ok, so I have found clarification for this [fake URL removed] where it says "The rules count surrounded points in seki, but not the 'neutral' points."
- BuggyMind: We don't need to appeal to fake URLs. Third sentence of rule 9: Any stones which the players agree could not escape capture if the game continued, but which have not yet been captured and removed, are termed dead stones. And according to rule 10, if the players can't agree on the status of stones, all stones on the board after both players pass twice in succession are alive. So seki is alive. Rule 12, Territory: Those empty points on the board which are entirely surrounded by live stones of a single color are considered the territory of the player of that color. Therefore, eyes in seki are territory.
- BuggyMind: Oh, and I found the clarification I think you meant, at http://www.cs.cmu.edu/~wjh/go/rules/AGA.commentary.html.
Anon Rule 10 states that 'If the players disagree about the status of a group of stones left on the board after both have passed, play is resumed, with the opponent of the last player to pass having the move. <snip>' In AGA rules White must always pass last, so why do the rules not state that Black moves first in the case of resumption?
Pledger: Under AGA rules, the first two consecutive passes end alternation, and the game moves to the agreement phase. The rule that "White must make the last move" is not applied until after the agreement phase. Therefore, if Black passed last before the agreement phase, the players wait to see if there will be resumed play. If afterwards, resumed play or not, Black has still made the last pass, White will then make an additional pass. See Equivalence Scoring example for a demonstration of the rule. (comment edited by author)
Anon Thank you for that answer, in practice I have always seen the third stone given over automatically and then the agreement phase begun. That it should be as you suggest is totally unclear from reading the AGA rules themselves. It strikes me as something that ought to be properly clarified in the rules.