Fail-Low

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Going down? [1]

There are two related semantics, if talking of a Fail-Low - inside the search tree or if using aspiration windows at the root.

Inside the Tree

A Fail-low appears at so called All-Nodes inside the alpha-beta algorithm, also called Fail-Low nodes. The score returned is a upper bound on the exact score of the node.

Quote by Bruce Moreland [2]:

A fail-low indicates that this position was not good enough for us.  We will not reach this position, because we have some other means of reaching a position that is better.  We will not make the move that allowed the opponent to put us in this position. 

Root with Aspiration

Another meaning of a Fail-Low, is associated with aspiration windows at the root, where one needs to re-search with wider windows to get the true score rather than an upper bound.

Bruce again:

You can also talk about failing high and failing low from the root position, if you use an aspiration window. 

See also

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