ECO codes are a classification system that sorts every chess opening into one of 500 codes, from A00 to E99, grouped under five letters: A, B, C, D, and E. “ECO” stands for the Encyclopaedia of Chess Openings, the reference work that created them. Each code labels a specific opening or family — so “C60” means the Ruy Lopez, “B20” the Sicilian, and so on. It’s basically the Dewey Decimal System for chess openings.
If you’ve ever seen “C50” next to a game and wondered what it meant, this is your decoder ring.
The 30-second version
- ECO = Encyclopaedia of Chess Openings, a system of 500 codes (A00–E99).
- Five letter groups: A, B, C, D, E, each covering broad opening types.
- Each code (like C50) pins down a specific opening or variation.
- It’s a filing system — useful for organizing study and databases, not for learning to play.
- You don’t need to memorize codes; just understand the map.
What does ECO stand for?
ECO is the Encyclopaedia of Chess Openings, a landmark reference first published in the former Yugoslavia in the 1970s. Before ECO, openings were named in a glorious mess of overlapping titles and languages. ECO imposed order by assigning every opening a tidy letter-plus-number code, and that system stuck — today nearly every chess database, book, and website uses it.
Think of it as chess’s universal cataloguing standard. When two players from different countries discuss “B90,” they both know exactly which Sicilian Najdorf line they mean, no translation needed.
How does the A–E system work?
The 500 codes are split into five big buckets by letter. Here’s the map:
| Group | Covers | Examples |
|---|---|---|
| A | Flank openings & others (1.c4, 1.Nf3, and irregular stuff) | English Opening, Réti, Dutch Defense |
| B | 1.e4 except 1…e5 and the French | Sicilian, Caro-Kann, Pirc, Scandinavian |
| C | 1.e4 e5, plus the French Defense | Italian, Ruy Lopez, French |
| D | 1.d4 d5 and the “Indian” defenses with …d5 | Queen’s Gambit, Slav, Grünfeld |
| E | Indian defenses with …e6 or …g6 (no early …d5) | Nimzo-Indian, King’s Indian |
Within each letter, the two-digit number narrows it down: C50 isn’t just “an e4 e5 opening,” it’s specifically the Italian Game’s main territory. The more you zoom in, the more precise the code gets.
A few codes you’ll actually run into
You don’t need to memorize these, but recognizing the big ones is handy:
- B20–B99 — the Sicilian Defense (a huge slice of group B; the Najdorf alone is B90–B99).
- C50–C54 — the Italian Game.
- C60–C99 — the Ruy Lopez.
- C00–C19 — the French Defense.
- D06–D69 — the Queen’s Gambit and its Declined lines.
- E60–E99 — the King’s Indian Defense.
- A00 — the catch-all for weird, rare first moves (the “everything else” drawer).
Notice how the Sicilian sprawls across eighty codes (B20–B99). That’s a clue to just how much theory it contains — and why it’s the most analyzed opening in chess.
Why do ECO codes matter?
For a casual player, mostly they don’t — and that’s fine. But they’re genuinely useful for three things:
- Organizing study. Databases let you filter by ECO code, so you can pull up every Najdorf (B90) game and study the pattern in one place.
- A common language. “I play the B12 Caro-Kann Advance” instantly tells another player exactly what you mean.
- Spotting depth. The number of codes an opening occupies hints at how much theory lurks inside it (Sicilian: 80 codes; the whole English: shares group A with others).
The big thing to internalize: ECO is a filing system, not a teacher. It tells you where an opening lives in the catalogue; it doesn’t tell you how to play it. For that, you still need to learn the ideas — which is what every individual opening guide on Chess Lab Academy is for.
Do I need to memorize ECO codes?
Nope. Absolutely not. No one’s quizzing you, and knowing “the Ruy Lopez is C60–C99” doesn’t make you a single rating point stronger. Understanding the ideas of the Ruy Lopez does.
Treat ECO codes the way you treat a library’s call-number system: handy for finding things, irrelevant to actually reading them. Recognize the five letter groups, know a couple of codes for your own openings so you can look them up in databases, and move on. The codes are scaffolding, not the building.
Common misconceptions about ECO codes
- “A lower number means an easier opening.” False. The numbers organize by move structure, not difficulty. A00 (rare first moves) isn’t “beginner-level.”
- “Each opening has exactly one code.” Many openings span a range (the Sicilian covers B20–B99) because they branch into so many variations.
- “Learning the codes teaches you the openings.” It doesn’t — codes are labels. You still have to learn the plans behind the moves.
- “ECO codes are official FIDE rules.” They’re a publishing classification, not part of the rules of chess. Useful convention, nothing more.
- “The codes never change.” They’re occasionally refined as theory evolves, but the framework has been remarkably stable since the 1970s.
FAQ
What does ECO stand for in chess? ECO stands for the Encyclopaedia of Chess Openings, a reference work first published in the 1970s that classifies every opening with a letter-and-number code from A00 to E99.
How many ECO codes are there? There are 500 ECO codes total, running from A00 to E99, split across five letter groups (A, B, C, D, E), each covering broad categories of openings.
What do the letters A, B, C, D, E mean? A is flank/irregular openings; B is 1.e4 lines other than 1…e5 and the French; C is 1.e4 e5 plus the French; D is 1.d4 d5 and …d5 Indian lines; E is Indian defenses with …e6 or …g6.
Do I need to learn ECO codes to get good at chess? No. ECO codes are a filing system for organizing and finding openings in databases. Improving comes from understanding opening ideas, not memorizing codes.
Why does the Sicilian have so many codes? The Sicilian (B20–B99) occupies 80 codes because it branches into an enormous number of variations — it’s the most analyzed and theory-rich opening in chess.
Keep climbing
Now that the map makes sense, go explore the territory. Pick a code group that matches your style and dive into a real opening — the ideas are where the strength actually comes from. Start with how to choose a chess opening, and remember it all sits on opening principles.
Next up: Browse the catalogue from the top — the chess openings hub links every family by name and code.
Written and fact-checked by the Chess Lab Academy Editorial Team (AI-assisted, human-edited) · Classification per the Encyclopaedia of Chess Openings; positions verified with Stockfish 17 · Last verified 2026-06-25 · Sources · How we verify content.