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I’ve read up on Babson tasks, and they mention that all known tasks are not perfect because there are duals. I’m not sure what this means exactly. What precisely is a “dual” in chess problems?

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A dual is an alternate solution, other than the intended one, after the first move of a problem or study. If it happens on the first move, the problem or study is called cooked instead.

dual

Ideally, White should have only one move at each juncture that solves a problem. If White has an alternative at any stage other than the first move, this is a dual. A dual is not as serious a flaw as a cook, and in minor lines, duals may be permissible (opinions differ on this point). Some problems make a virtue out of dual avoidance – of two apparently equivalent white moves, only one works.

(source: Wikipedia)

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  • Wiki misses a slight point: If Black ignores a threat, this is not seen as a dual...usually. (In olden times, one was less lenient about it.) E.g. Kc8 Rb5 Bg1 - Ka8 Pa7. Black has no defense against Rb8#, so is 1...a5 2.Rxa5 a dual? Mar 17, 2021 at 8:01
  • That may be what the article means by 'minor lines'?
    – Glorfindel
    Mar 17, 2021 at 8:11
  • I'd say the case I describe is a subset of 'minor lines'. I'd define a minor line as anything that is not thematic. A dual minor is still a thing you should strive to avoid nowadays, whereas hardly anyone cares about a dual by ignorance. Mar 17, 2021 at 9:20

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