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The ECO Code Index: A Guide to Opening Classifications (A00-E99)
If you look at a chess database, you will see cryptic codes like B90 or E15 next to the opening name. These come from the Encyclopedia of Chess Openings (ECO), the standard system for classifying every possible chess opening into five volumes (A through E).
Volume A
Flank Openings & Irregular Lines
A00 Irregular Openings (Grob, Polish)
A02-A03 Bird's Opening (1. f4)
A04-A09 RĂ©ti Opening
A10-A39 English Opening (1. c4)
A40-A44 Queen's Pawn (1. d4 without 1...d5)
A50-A79 Benoni & Benko Gambit
A80-A99 Dutch Defense
Volume B
Semi-Open Games (Sicilian, Caro-Kann)
B00-B05 Alekhine's Defense
B06-B09 Robatsch & Pirc Defense
B10-B19 Caro-Kann Defense
B20-B29 Sicilian: Closed / Anti-Sicilians
B30-B39 Sicilian: Accelerated Dragon / Kalashnikov
B40-B49 Sicilian: Taimanov / Kan / Paulsen
B70-B79 Sicilian: Dragon Variation
B90-B99 Sicilian: Najdorf Variation
Volume C
Open Games (1.e4 e5) & French Defense
C00-C19 French Defense
C20-C29 King's Pawn (Vienna, Bishop's)
C30-C39 King's Gambit
C42-C43 Petroff Defense
C44-C45 Scotch Game
C50-C59 Italian Game & Two Knights
C60-C99 Ruy Lopez (Spanish Game)
Volume D
Closed Games (1.d4 d5) & GrĂĽnfeld
D00-D05 Queen's Pawn (London, Colle)
D06-D09 Queen's Gambit (Chigorin, Albin)
D10-D19 Slav Defense
D20-D29 Queen's Gambit Accepted (QGA)
D30-D69 Queen's Gambit Declined (QGD)
D70-D99 GrĂĽnfeld Defense
Volume E
Indian Defenses (1.d4 Nf6)
E00-E09 Catalan Opening
E12-E19 Queen's Indian Defense
E20-E59 Nimzo-Indian Defense
E60-E99 King's Indian Defense (KID)