Ladder

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  Difficulty: Introductory   Keywords: Tesuji, Go term

Chinese:

  • 扭羊头 (niǔ yáng tóu)
  • 征 (zhēng);
  • 征子 (zhēng zǐ)

Japanese: シチョウ (shichō)

  • 征 (shichō) (Rare)

Korean: 축/逐 (chuk)

A ladder is a technique for capturing stones. At each step the attacker reduces the defender's liberties from two to one.[1]

The term ladder may also refer to a ranking system used within clubs. See Club Ladder for details.


Example

[Diagram]

Ladder

Black B1 starts a ladder to capture the white+circle stone.

[Diagram]

Playing out the ladder

If White tries to escape, Black can at each time play atari and White will be captured.

Clearly White should see this and not play W2.


Long ladder

[Diagram]

A long ladder

Ladders may span a large part of the board. If for example Black captures the white stone in a ladder with B1, the ladder will go all over the board if White tries to escape, resulting in the next diagram.

[Diagram]

A long ladder

If White tries to escape, the ladder will continue in this way. Of course, as it stands, this is a disaster for White.

But if there had been a white stone at a, White could now capture a stone and would stand to capture more because of all the double ataris that White can play. Such a stone which stops a ladder from working is called a ladder-breaker.


Linguistic note on Chinese

The Chinese term for "ladder", 征子 (zhēng zǐ), came from the term 长征 (cháng zhēng), which means "long march".


Computational complexity of ladders

Marcel Crasmaru and John Tromp proved that the [ext] problem of computing ladders belong to the class of [ext] PSPACE-complete, due to the possible twists and turns and other complications in the execution of a ladder. However, such complicated cases are quite rare in Go. The majority of the ladders can be easily read out by a beginner with some practice. (A similar comment applies to many such proofs. Indeed, they are "asymptotic" and really only apply to arbitrarily large boards. By themselves they say nothing about our good old 19x19 goban. Concrete bounds are needed for that.)


See also:

Homonym


[1] The term, "ladder", though long in use, is also debatable. The term "stair" is much better to describe this form. (But... A stair and not a ladder, a thing and not a process? It's a reckless path to disaster, not a trip to the attic.)


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