By default, we start searching a position with alpha and beta set to negative infinity and infinity respectively (a so-called infinite window).

Using an aspiration search, we make a guess as to the score that will be returned, say x, and set alpha and beta to x-d and x+d respectively, where d is some integer.

• The guessed value of x is typically the score returned by the previous iteration of the search.
• In order to provide an estimate to start with, the first few iterations are typically done with an infinite window.
• If the true score is indeed within the narrowed search window, we will have saved time due to the additional cutoffs obtained.
• If our guess was wrong, however, the score returned will be useless, and we must waste time searching the position again with an infinite window to obtain the true score.
• The value of d should be set as small as possible while avoiding too many researches.
• The optimal value will vary between programs, but typically a value of 50 centipawns is used.

Aspiration Search