Ko (1)

	Ko
	In Dia. 1 below, one Black stone is in atari. White can capture it, as shown in Dia. 2.
	Dia. 1	Dia. 2
	Please have a good look at the shape after White captures the black stone (Dia. 3.). Now one White stone is in atari. Since it's Black's turn to play, how about capturing the stone, as in Dia. 4?
	Dia. 3	Dia. 4
	Eh? Dia. 4 is the same as Dia. 1. We're back in the original position. It's White's turn to play next, but if he captures Black, then Black recaptures, etc., we'll just get the same positions being repeated again and again. If that happens, the game will never end.
	This position, in which both sides can go on endlessly capturing a stone, is called  'ko'.
●	The ko rule
	'Ko'
	According to the ko rule, in a position where White has captured a single stone, as in Dias. 1 and 2 above, Black cannot immediately recapture (as in Dia. 4). In other words, repeating the same shape is prohibited.
	Since Black can't immediately recapture, he must first play a move elsewhere (called a 'ko threat'). After that, he can then recapture the ko.
	Dia. 5
	To show how this works, in Dia. 5, which is a ko, Black can't immediately capture a stone by playing at the x point, so he must play one move elsewhere, for example, Black 1. Now he can recapture the ko on his next move.

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