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A ladder shape for black (it is white's move). Black will win this ladder battle due to the marked black stone, which will put the white stone that will be played at the spot indicated by a into atari; if the marked stone did not exist, then white would inevitably win when the regular pattern of play extended to the edge of the board. A ladder is full of opportunities for a double atari on white, such as positions c and d.

In the game of Go

Go (board game)

Go , is an ancient board game for two players that originated in China more than 2,000 years ago...

, a
is a basic sequence of moves in which an attacker pursues a group in atari in a zig-zag pattern across the board. If there are no intervening stones, the group will hit the edge of the board and be captured.

The sequence is so basic that there is a Go proverb

Go proverb

Go proverbs are traditional proverbs relating to the game of Go, generally used to help one find good moves in various situations during a game. They are generalisations and thus a particular proverb will have specific situations where it is not applicable. Knowing when a proverb is inapplicable is...

saying "if you don't know ladders, don't play Go."

The ladder tactic is often a failure, if there are stones supporting those being chased close enough to the diagonal path of the ladder. Such a failing ladder is called a broken ladder. Secondary double threat tactics around ladders, involving playing a stone in such a way as to break the ladder and also create some other possibility, are potentially very complex. Such plays are called ladder-breakers.