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What is an ECO code in chess?

The Encyclopaedia of Chess Openings (ECO) is a classification system for chess openings. Each opening is assigned a code from A00 to E99, organized into five main volumes:

A - Flank openings (English Opening, Reti, Bird's Opening) B - Semi-open games (Sicilian Defense, French Defense, Caro-Kann) C - Open games (1.e4 e5 lines like Ruy Lopez, Italian Game, Scotch) D - Closed games and Semi-closed (Queen's Gambit, Slav, London System) E - Indian defenses (King's Indian, Nimzo-Indian, Queen's Indian)

ECO codes help players quickly identify and study specific opening variations.

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