
# Glossary

## Book

When humans play chess, they don't think about their first few moves, because they have figured them out in advance.  A great deal of effort has been expended by people who write books about the opening moves of chess, and people read these books, memorize sequences, and hopefully understand why these sequences are good or bad.

It is not necessary that a computer chess program generate its first moves via the search process.  In almost every case that I can think of, the programmer has built an "opening book".  At the start of the game, the program looks moves up in this book and simply makes them without thinking.

## Fail Low, Fail High

An alpha-beta search is guided by two bounds, which are called alpha and beta.  Alpha is always less than beta.  Each successor position is searched.  If a successor's value is greater than or equal to beta, the search will stop and return beta at that point.  This is called a "fail high".  If all of the successors are searched, and none of them returns a score in excess of alpha, the search will return alpha.  This is called a fail low.

A fail-high indicates that the search found something that was "too good".  What this means is that the opponent has some way, already found by the search, of avoiding this position, so you have to assume that they'll do this.  If they can avoid this position, there is no longer any need to search successors, since this position won't happen.

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.

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

## Successor position

A successor position is a position that can be reached from the current position by a legal move.  Meaning, that if you set the pieces up in order to play a game of chess, if you are playing white, your job is to choose amongst various successor positions, by making a legal move.  If you play 1. e4, you have achieved this, and the new position is a successor of the start position.


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Copyright © 2001 Bruce Moreland
Last modified: 01/24/03
