Opening Book

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Home * Knowledge * Opening Book

Chess programs often look up the positions at the beginning of the game in an Opening Book. The opening database can then be used as long as the opponent plays a new move from the database, so usually more common opening lines will be stored to a much higher depth than the uncommon ones. As soon as the program is "out of book" it has to continue using the normal search routines.


  • Save time: Chess programs could use search from the beginning and look through all possible continuations in detail to find the best next move, but as this is quite time-consuming and there already exists a lot of literature about different opening lines, opening books save time.
  • Provide a higher quality of play: Searching only even with large depths can't see deep tactics, strategy, compensation, lose temporarily material for long term advantage.
  • Provide variety: as book moves are usually chosen randomly, whereas searches are more or less deterministic.


Opening books are typically stored in two formats:


The most advantage is that by using text form they are readable, understandable, and editable by humans without any special software. However, the main disadvantage is that they may take too much space as well as time for searching since an item may take from 50 bytes (for FEN strings) to hundreds of bytes (for PGN text). If we store openings "continuously" (all possible opening positions) both data size and searching time become too huge and become unacceptable for playing. In practice, those opening books are usually very small with some lines or positions only, they cannot be used for real game playing but for testing.

They are stored in typical ways for games and positions:

  • EPD: They are the last positions of opening lines
  • PGN: Opening lines are stored as individual games

(e4(e5)(c5))(d4d5)) etc.


Binary books have some important advantages, especially for computer chess engines. Opening databases built on all positions are usually more space-efficient and overcome the problem of transpositions. Often these positions are stored as hash values to allow fast access. Some opening database formats store with each position as well as some information about it. (e.g. number of times occurred, number of games won by white/black / drawn with this position, average/maximum Elo of players playing to this opening position, chess program's success with the position). So when a chess program has to decide on a move, it will look up all positions that could arise after moving in the database and then make a decision upon the information given with each position. Depends on designs, an item may take 10 - 16 bytes. To build and/or edit those opening books, users need to use special software.

The most popular format is PolyGlot.


All in all the information stored in an opening database should guide the chess game into a type of middle game in which the chess program proves itself most successful. But to achieve this there are two main ways.


If the opening database designer has some special preference of a certain opening line, he might create it fully manually. This is often the case if a chess program is about to play an important game against a grandmaster. Then the opening style will probably be chosen to be difficult and dangerous for the opponent. Another option is to first automatically generate a database and then fine-tune it manually.

From game collection

Another option is to select a number of games (the type of which will affect the openings played: e.g. drawn games only, GM games only, etc) and then up to a certain depth store all positions in the opening database. It is true that by using this way of database generation some severe errors might by copied as well, but usually the chess programs are set to stop playing according to the book if a position occurred less than a certain number times before.

By computing

Chess engines can be used to evaluate positions in a book opening tree to rank them, and/or decide to stop or expand.

Book Building Tools


Jonathan Schaeffer in One Jump Ahead [5] :

To solve the opening problems of his chess machine, Belle, Ken Thompson typed in opening lines from the Encyclopedia of Chess Openings (in five thick volumes). Religiously, he dedicated one hour a day for almost three years (!) to the tedious pursuit of entering lines of play from the books and having his Belle computer verify them. The result was an opening library of roughly three-hundred thousand moves. The results were immediate and obvious: Belle became a much stronger chess program, and Ken probably aged prematurely. Later Ken developed a program to automatically read the Encyclopedia, allowing him to do in a few days what had taken him three years to do manually. [6] 


Classification of Chess Openings

Book Issues

See also

Selected Publications

1974 ...

Я.Ю. Гольфанд, А.Л. Футер (1974). Реализация дебютной справочной для шахматной программы./ Сб. Проблемы кибернетики №29, стр. 201-210

1980 ...

1990 ...

2000 ...

2005 ...

2010 ...

2015 ...

Forum Posts

1990 ...

1995 ...

2000 ...

2005 ...

Re: Opening books format by Jury Osipov, CCC, April 15, 2008 » ABK
Re: Opening books format by Richard Pijl, CCC, April 15, 2008
Re: Opening books format by Dann Corbit, CCC, April 15, 2008

2010 ...


Re: 2 Moves Engine Book by Miguel A. Ballicora, CCC, December 08, 2013 » Ruby


2015 ...





2020 ...

External Links

Engine Books

Online Opening Tree



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