Rule 4: A stone may not be played if it will make the board look exactly the same as it did earlier in the game. (Repetition is not allowed.)
This rule prevents the same moves from being played over and over again
so that the game never ends. In real games, there is only one common pattern
for which this rule is needed. It is called a "ko" (which rhymes
with "go") and it looks like this:
Black can play 1, which captures a white stone. But if White played 2,
capturing the stone that Black just played, it would bring us right back
where we started. Black and White could go on capturing each others' stones
forever. So White is not allowed to play stone number 2 right away. Instead,
White must first play a move somewhere else on the board. If Black plays
somewhere else, too, then White can come back and play stone number 2 because
this will not make the board look the same as it did before (the two stones
that were played somewhere else will make it look different).
In Figure 10, below, Black has just captured a white stone by playing 1. White cannot recapture 1 right away, and must look for a move to play somewhere else. White would like to find a move that threatens some black stones, so that Black will have to play somewhere else, too. So White plays 2 and says "atari". This move is called a ko threat. Now Black could fill in the ko, but then two black stones would be captured, so Black plays 3 to save them. White then takes the ko with 4. Now it is Black who will have to find a ko threat. This fight can go on for quite a long time, and it is called a ko fight. But sooner or later, one of the players will end the ko, usually by filling it in and saving the stone that is in atari.
Ko fights can happen on the edge of the board or in the corner, too.
The pattern is the same:
Here is a complete ko fight that happens at the end of a game on a very
small 7x7 board. In Figure 12, Black has just played 1, and now White can
play 2, capture a black stone, and start a ko fight. Black plays 3 as a
ko threat, putting 2 white stones into atari. White saves them by playing
4 which connects them to some other white stones. Now Black can take back
the ko with 5.
Now White plays 6 in Figure 13 below, which is a good ko threat since
it puts seven black stones into atari. Black saves them by playing 7, and
White takes back the ko with 8. Now Black has run out of threats. Black
plays 9, but White can just ignore it and fill in the ko with 10, ending
the ko fight. (The game is now over, too.)
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