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What Is an ECO Code in Chess?
An ECO code is a short label — a letter from A to E followed by two digits — that classifies a chess opening. It comes from the Encyclopaedia of Chess Openings, and codes like B90 for the Sicilian Najdorf give every opening a compact, standard name.
How the coding works
The five letters split all openings into broad families: A covers flank openings, B and C cover different branches starting with 1.e4, and D and E cover openings starting with 1.d4 and related systems. The two digits that follow narrow things down to a specific variation within that family, so B90 sits precisely within the Sicilian Defense family covered under B.
Why chess uses this system
Opening names in everyday speech can be long, inconsistent, or overlap between languages — 'the Sicilian, Najdorf, English Attack' is a mouthful. An ECO code gives databases, engines, and players a single unambiguous tag for any opening, which makes searching game databases and studying opening statistics far easier.
Where you'll see ECO codes
Game databases, chess engines, and tournament software commonly tag every game with its ECO code the moment the opening moves are played. If you're browsing your own games or studying a database of grandmaster games, filtering by ECO code is one of the fastest ways to find every game that started with a particular opening.
Frequently asked questions
What does ECO stand for?
Encyclopaedia of Chess Openings, the reference work that introduced this classification system.
What is B90 an example of?
B90 is the ECO code for the Sicilian Najdorf, a specific and heavily studied variation within the Sicilian Defense.
Do all openings get their own unique code?
Related lines within a family often share a code, with further move-by-move detail distinguishing sub-variations in fuller reference works — but every major opening family has its own code range.