How the 500 ECO codes are organised
Five letters, a hundred numbers each, and what the Lichess opening list files under them.
The ECO system splits chess openings into five volumes, A to E, each divided into 100 codes, so every opening lands on one of 500 codes from A00 to E99. The letter tells you the broad family of first moves; the number narrows it down. In the list behind this site, volume C is the fullest with 1,250 named lines and volume E the leanest with 367.
Five volumes, one hundred codes each
A code is a shelf, not an opening. Some shelves hold a single named line; A00 holds 144. The letter is set by the opening moves. This is how the five volumes compare in the Lichess list:
| Volume | Heading | Lines | Openings | Codes with one line | Busiest code | Median length (plies) |
|---|---|---|---|---|---|---|
| A | Flank openings | 823 | 67 | 26 | A00 (144) | 6 |
| B | Semi-open games | 781 | 25 | 20 | B00 (83) | 8 |
| C | Open games and the French Defense | 1,250 | 30 | 6 | C44 (53) | 9 |
| D | Closed and semi-closed games | 644 | 13 | 18 | D00 (72) | 11 |
| E | Indian defenses | 367 | 9 | 30 | E14 (22) | 12 |
Two patterns stand out in that table. The first is size. Volume C, which collects the open games after 1.e4 e5 and the French Defense, files 1,250 lines under its hundred codes, 3.4 times the 367 of volume E. The second is depth. The median line in volume A is 6 plies long, in B 8, in C 9, in D 11 and in E 12. Every step from A to E adds length, so in volume E a name is typically attached later in the game than anywhere else.
Volume A: everything that is not a main road
Volume A is the catch-all. It holds 67 different opening names, far more than any other volume, and its lines start with 20 different first moves. The most frequent is 1.d4 with 329 lines: these are the queen’s pawn openings filed in A, among them the Dutch Defense (69 lines) and the Benoni Defense (66). Then come 1.c4 (198), the English Opening, and 1.Nf3 (103). The rest are rare first moves gathered under A00, from 1.b4 to 1.Nh3.
Because so many short openings live here, A is also where you find the shortest lines. Twenty-six of its codes hold one line only.
Volumes B and C: the two halves of 1.e4
Volumes B and C share the first move. Of the 781 lines in B, 779 start with 1.e4; in C the figure is 1,248 of 1,250. What separates them is Black’s reply. In volume C the commonest answers are 1...e5 (1,033 lines) and 1...e6 (212), which is why the French Defense sits next to the open games. Volume B gathers the other replies: 1...c5 leads with 409 lines, followed by 1...c6 (110), 1...Nf6 (56) and 1...d5 (46).
The busiest code of volume B is B00, with 83 lines spread over many small defenses; in volume C it is C44, with 53.
Volumes D and E: the queen’s pawn
Almost every line in volumes D and E opens with 1.d4: 636 of 644 in D and 365 of 367 in E. Volume D covers the closed games after 1.d4 d5, including the Queen’s Gambit lines, plus the Grünfeld Defense. Volume E is the home of the Indian defenses that begin 1.d4 Nf6 2.c4 e6 or 2...g6: the Nimzo-Indian, the Queen’s Indian, the Bogo-Indian and the King’s Indian.
Volume E has the fewest lines but the highest number of single-line codes, 30. Many of its codes are reserved for one precise branch of the King’s Indian or Nimzo-Indian, deep in the game.
Codes that mix several names
Most codes hold lines of one opening name only, but not all. The number of codes in each volume whose lines carry two or more names is A 21, B 6, C 13, D 8, E 3. Volume A again leads, because its codes for rare first moves gather many small, unrelated openings. In D and E, where long main lines are subdivided move by move, a code usually belongs to a single name.
Why a code is not the same as an opening
The code and the name answer different questions. A name such as Sicilian Defense covers 400 lines in the list and spreads over 80 codes, while one code such as A00 collects 20 different names. On this site, every code has a page with the board it reaches and its lines, and every name that spans several codes has an opening page showing which codes it occupies.
To place a game of your own, the opening finder reads its moves and reports the deepest named position, with its code.
Sources
Opening names and move sequences: lichess-org/chess-openings (CC0), retrieved 2026-10-28. All counts replayed and computed by ecocodes with python-chess; definitions on How to read a code page.