Editing Computer chess (section)

== Opening book ==
Chess engines, like human beings, may save processing time as well as select variations known to be strong via referencing an [[Chess opening book|opening book]] stored in a database.  Opening books cover the opening moves of a game to variable depth, depending on opening and variation, but usually to the first 10-12 moves (20-24 ply).  In the early eras of computer chess, trusting variations studied in-depth by human grandmasters for decades was superior to the weak performance of mid-20th-century engines.  And even in the contemporary era, allowing computer engines to extensively analyze various openings at their leisure beforehand, and then simply consult the results when in a game, speeds up their play.

In the 1990s, some theorists believed that chess engines of the day had much of their strength in memorized opening books and knowledge dedicated to known positions, and thus believed a valid [[anti-computer tactics|anti-computer tactic]] would be to intentionally play some out-of-book moves in order to force the chess program to think for itself.  This seems to have been a dubious assumption even then; Garry Kasparov tried it via using the non-standard [[Mieses Opening]] at the 1997 [[Deep Blue versus Garry Kasparov]] Game 1 match, but lost.  This tactic became even weaker as time passed; the opening books stored in computer databases can be far more extensive than even the best prepared humans, meaning computers will be well-prepared for even rare variations and know the correct play.  More generally, the play of engines even in fully unknown situations (as comes up in variants such as [[Chess960]]) is still exceptionally strong, so the lack of an opening book isn't even a major disadvantage for tactically sharp chess engines, who can discover strong moves in unfamiliar board variations accurately.

In contemporary engine tournaments, engines are often told to play situations from a variety of openings, including unbalanced ones, to reduce the draw rate and to add more variety to the games.<ref>{{Cite web |title=TCEC Openings FAQ |url=https://tcec-chess.com/articles/TCEC_Openings_FAQ.html |access-date=2023-10-12 |website=tcec-chess.com}}</ref>