The Encyclopaedia of Chess Openings (ECO) is a book collection (now also a computer database) describing chess openings. The moves were taken from hundreds of thousands of games between masters, from published analysis in the Chess Informant since 1966, and then compiled by notable chess players. Both the ECO and the Chess Informant are published by the Serbian company Šahovski informator sahovski.

Instead of the traditional names for the openings, ECO has developed a unique coding system that has also been used by other chess publications. There are five main categories, A-E, each of which is divided into a hundred sub-categories.

Main ECO codes

A

1. R' 1. e4, 1. d4: Irregular openings.
2. d4 R' 1 ...d5, 1... Nf6: Atypical replies to 1.d4
3. d4 Nf6 2. R' 2. c4: Atypical replies to 1...Nf6
4. d4 Nf6 2. c4 R' 2... e6, 2... g6: Atypical Indian systems.

B

1. e4 R' 1... c5, 1... e6, 1... e5: Most Semi-Open Defences
2. e4 c5: Sicilian Defence

C

1. e4 e6: French defence
2. e4 e5: Double King Pawn games

D

1. d4 d5: Double Queen Pawn games
2. d4 Nf6 2. c4 g6 + d7-d5: Grünfeld Defence

E

1. d4 Nf6 2. c4 e6: Indian systems with ...e6
2. d4 Nf6 2. c4 g6 ' d7-d5: Indian systems with ...g6 (except Grünfeld)

Also see

• List of chess openings for the full ECO codes