Definition: Encyclopedia of Chess Openings; a multi-volume reference published by Chess Informant. ECO classifies openings using codes from A00 to E99. The codes are in wide use throughout the chess world.
As of February 2004, Chess Informant described the ECO in the following way : 'A mini-library of five volumes (A, B, C, D, E - referring to the five main groups of the classification of openings) provides readers with a comprehensive collection of modern opening theory. On over 2300 pages the examples from over 120.000 games and analyses are included. Contributors and authors of this important working tool are superclass chess players from all over the world'
