ECO= Encyclopedia of Chess Openings,which used to publish a 5 volume set of chess opening theory but hasn't published any new volumes since 2008. It still publishes the Chess Informant, which publishes many leading grandmasters games with annotations by the players.
ECO set up a classification system for all openings, so that you can easily identify what kind of opening was used. Chess.com's book openings list the ECO code for various openings. For example, the ECO code for the Alapin variation of the Sicilian defense is B22.
Here are the descriptions of the five letter codes:
| A | English Opening, Reti, Benoni, Volga Gambit, Dutch Defense, ... |
| B | Sicilian Defense, Caro-Kann, Pirc, Modern, Alekhine, Scandinavian, ... |
| C | Ruy Lopez, French, Scotch, Russian, King’s Gambit, ... |
| D | Queen’s Gambit, Slav, Gruenfeld, Queen's Pawn Game, ... |
| E | King’s Indian, Nimzo-Indian, Queen’s Indian, Bogolubov, Catalan, ... |
What does ECO mean?