In short: A system that divides the chess openings into five groups, A to E, numbered from 00 to 99. The Sicilian Defense, for example, has the codes B20 to B99.
Also called ECO code.
ECO stands for Encyclopaedia of Chess Openings, a five-volume encyclopedia of chess openings. It was produced in Yugoslavia in the 1970s, and its way of numbering the openings has become the standard. An ECO code is a letter from A to E, giving the volume, and a number from 00 to 99.
The five groups
| Volume | Contents | For example |
|---|---|---|
| A | Flank openings and unusual openings | 1. c4 (English Opening), the Benoni, the Dutch Defense (1. d4 f5) |
| B | Semi-open games except the French | 1. e4 c6 (Caro-Kann), 1. e4 c5 (Sicilian Defense, B20–B99) |
| C | Open games and the French | 1. e4 e6 (French Defense), 1. e4 e5 (Ruy Lopez and Italian Game) |
| D | Closed games | 1. d4 d5 (the Queen's Gambit), the Grünfeld Defense |
| E | Indian defenses | the Nimzo-Indian, Queen's Indian and King's Indian Defense |
The codes follow the moves
The ECO codes are based on the opening moves, not on the names of the openings. A big opening can therefore have many codes: the Sicilian Defense has the whole range B20 to B99. And one code can hold several variations with names of their own.
On our pages about chess openings the code is given for every opening and for many of the variations. Read also about opening theory and variations.
Frequently asked questions
What does ECO stand for?
Encyclopaedia of Chess Openings, a five-volume encyclopedia of openings produced in Yugoslavia in the 1970s.
Does every opening have its own code?
No. The codes follow the moves, not the names. A big opening can have many codes, and one code can hold several variations.
Updated November 14, 2026