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ECO coding is a unique system of classifying all chess openings by means of 500 codes (from A00 to E99). It first appeared in 1966 in the initial publication of Chess Informant (a regular analysis periodical now approaching its 100th issue), and was then used as the classification basis in an Encyclopedia of Chess Openings, a virtually languageless reference work first published in 1974-9 by Chess Informant in 5 volumes (A to E, each covering one classification group). This led to it becoming known as the ECO system, now a FIDE-accepted standard classification used worldwide.

Revised volumes of the Encyclopedia of Chess Openings are published from time to time, providing a comprehensive resource collection of modern opening theory. Its 3,000 pages give examples from over 150,000 games and contain analyses contributed by top chess players from all over the world.

At its most basic level, ECO uses five code groups to cover all start-moves, described algebraically as:

Group A:
    - 1. excluding 1.e4, 1.d4
    - 1.d4 excl. 1..d5, 1..Nf6
    - 1.d4 Nf6 excl. 2.c4
    - 1.d4 Nf6 2.c4 excl. 2..e6, 2..g6
Group B:
    - 1.e4 excl. 1..c5, 1..e6, 1..e5
    - 1.e4 c5
  Group C:
    - 1.e4 e6
    - 1.e4 e5
Group D:
    - 1.d4 d5
    - 1.d4 Nf6 2.c4 g6 plus..d7-d5
Group E:
    - 1.d4 Nf6 2.c4 e6
    - 1.d4 Nf6 2.c4 g6 minus..d7-d5

Each code group has "divisions and sub-divisions of 10" whereby for example group A has categories A0-A9, and category A0 expands into codes A00 to A09. Thus group D ("1.d4 with early ..d5") includes a category D2 (classified as "1.d4 d5, 2.c4 dxc4", the Queen's Gambit Accepted), which in turn includes ECO code D22 ("1.d4 d5, 2.c4 dxc4, 3.Nf3 a6, 4.e3", the Alekhine Variation and off-shoots). All opening moves are thus categorised under one of the 500 ECO codes, with the more popular openings and defences found under certain "ranges" of codes:

ECO code ranges: A00-A39= FLANK OPENINGS (neither 1.d4 nor 1.e4) - includes English Opening
A40-A44= QP OPENING without 1..d5 or 1..f5 or 1..Nf6 reply
A45-A49= QP OPENING, INDIAN DEFENCE (1.d4 Nf6) without 2.c4
A50-A79= QP OPENING, INDIAN DEFENCE with 2.c4 but without 2..e6 or 2..g6
A80-A99= QP OPENING with 1..f5 reply (= Dutch Defence)
B00-B19= KP OPENING without 1..c5, ..1.e6 or ..1.e5 - incl. Alekhine, Caro-Kann Defs.
B20-B99= KP OPENING with 1..c5 reply (= Sicilian Defence)
C00-C19= KP OPENING with 1..e6 reply (= French Defence)
C20-C99= KP OPENING with 1..e5 reply (= Mutual King's Pawn, incl. Ruy Lopez)
D00-D05= QP GAME (1.d4 d5) without 2.c4 (= Queen's Pawn Game)
D06-D69= QP GAME (1.d4 d5) with 2.c4 (= Queen's Gambit)
D70-D99= QP, INDIAN DEF. (1.d4 Nf6, 2.c4 g6) with early ..d5 (= Grunfeld Defence)
E00-E59= QP, INDIAN DEF. (1.d4 Nf6) with 2.c4 e6 - includes Catalan Opening
E60-E99= QP, INDIAN DEF. (1.d4 Nf6, 2.c4 g6) without early ..d5 (= King's Indian)

Below is a summary shortlist of the main ECO codes, with links to some of the equivalent openings (NB: information is from various sources in Dec.2007 and has not been fully checked yet for 100% accuracy, being work still in progress Dec. 2009!):

ECO code
A00-A39Flank Openings
A00Irregular (Benko, Dunst, Polish etc)
A01Larsen Opening
A02-A03Bird's Opening
A03Dutch Variation
A04-A09Reti Opening
A06incl. Old Indian Attack
A07-A08Barcza System
A08incl. Grünfeld Reversed and King's Indian Attack
A09Landstrasse Gambit
A10-A39English Opening
A11-A12Anglo-Slav Defence (Caro-Kann System)
A13-A14Anglo-French Defence
A15-A19Anglo-Indian Defence
A17incl. Hedgehog Variation
A18-A19Mikenas-Carls Variation
A20-A29Sicilian Variation
A22-A24Bremen System
A25-A29Reversed Sicilian
A28-A29Four Knights System
A30-A39Symmetrical Variation
A33Symmetrical Four Knights Variation
A37-A39Ultra Symmetrical Defence
A40-A44Queen's Pawn, Irregular Defences
A40incl. English, Polish, Keres Defences, Englund Gambit
A41-A42Old Indian Defence
A43-A44Old Benoni Defence
A45-A79Indian Defences
A45incl. Trompowsky Attack
A46incl. Torre Attack
A47West Indian Defence
A48-49East Indian Defence
A50incl. Black Knights Tango
A51-A52Budapest (Counter Gambit) Defence
A52Normal Variation
A53-A55Old Indian Defence
A56Czech Benoni Defence
A57-A59Benko (Volga) Gambit
A58-A59Benko G. Accepted
A60-A79Modern Benoni Defence
A66-A69Pawn Storm Variation
A70-A79Classical Variation
A80-A99Dutch Defence
A82-A83Staunton Gambit
A87-A89Leningrad System
A92-A95Stonewall System
A97-A99Ilyin-Genevsky Variation
B00Irregular KP Defences incl. Nimzowitsch and Owen
B01Center-Counter (Scandinavian) Defence
B02-B05Alekhine's Defence
B03incl. Four Pawns Attack
B04-B05Modern Variation
B06Modern (or Robatsch) Defence
B07-B09Pirc Defence
B08Classical Variation
B09Austrian Attack
B10-B19Caro-Kann Defence
B11incl. Two Knights Variation
B13-B14Exchange Variation
B18-B19Classical Variation
B20-B99Sicilian Defence
B20incl. Sicilian Centre Game
B21Philidor Variation
B22Alapin Variation
B23-B26Closed Variation
B23incl. Grand Prix Attack
B27incl. Hungarian Variation
B28O'Kelly Variation
B29Rubinstein Variation
B30-B39Orthodox Variation
B30-B31incl. Rossolimo Variation
B32incl. Lowenthal (Hunting?) Variation
B33incl. Pelikan and Sveshnikov Variations
B34-B39Accelerated Dragon Variation
B36-B39Maroczy Bind
B40Counter Attack Variation
B41-B43Paulsen (Kan) Var.
B44Tiamanov System
B45Classical System
B47-B49Bastrikov (Tiamanov) System
B50incl. Kopec Variation
B51-B52Moscow Variation (Rossolimo Attack)
B53incl. Checkhover (Hungarian) Variation
B55incl. Venice Attack (Anti-Dragon Variation)
B57Sozin-Benko Variation
B58-59incl. Boleslavsky Variation
B60-B69Richter Rauzer Attack
B70-B79Dragon Variation
B71Levenfish Attack
B72Classical System
B75-B79Dragon-Sämisch Variation (Yugoslav Attack)
B80-B89Scheveningen Variation
B81Keres-Panov System
B86-B89Fischer/Sozin System
B90-B99Najdorf Variation
B96Goteborg System
B97Poisoned Pawn System
ECO code
C00-C19French Defence
C01incl. Exchange Variation
C02Advance Variation
C03-C09Tarrasch Variation
C03incl. 3..a6 System
C043..Nc6 System
C05-C063..Nf6 System (Closed Tarrasch)
C07-C093..c5 System (Open Tarrasch)
C10-C19Paulsen Variation
C11incl. Svenonius Variation and Steinitz Variation
C12incl. McCutcheon Variation
C13Burn Variation
C14incl. Classical Variation
C15-C19Winawer Defence
C15incl. Fingerslip Variation
C16-C19Advance Variation
C17incl. Bogoljubow Variation
C19Smyslov Variation and Positional Variation
C20-C99Mutual King's Pawn Openings
C21-C22Centre Game
C21incl. Danish (Nordic) Gambit
C223.Qxd4 Variation
C23-C24Bishop's Opening
C23incl. Classical Variation
C24Berlin Defence
C25-C29Vienna Game
C25incl. Vienna Gambit
C26-C29Falkbeer Variation
C27-C28Stanley Variation
C30-C39King's Gambit
C30KGD Classical Defence
C31-C32Falkbeer Counter Gambit
C33-C39King's Gambit Accepted
C33incl. Keres Gambit
C35Cunningham Variation
C36Abbazia Defence
C37-C39Classical Variation
C37incl. Muzio Gambit
C38incl. Hanstein & Philidor Gambits
C39incl. Allagier & Kiesertisky Gambits
C40-C99King's Knight Opening
C40incl. Latvian Counter-Gambit
C41Philidor's Defence
C42-C43Petroff's (Russian) Defence
C42Main line
C43Steinitz Attack
C44-C45incl. Scotch Game and Ponziani Opening
C44incl. Scotch Gambit
C45incl. (Scotch) Classical Variation
C46Three Knights' Opening
C47-C49Four Knights' Game
C48-C49Lopez Four Knights
C49Double Ruy Lopez Opening
C50incl. Hungarian Def.
C50-C54Giuoco Piano (Italian Game)
C50incl. Pianissimo Variation
C51-C52Evans Gambit
C53-C54Giuoco main line
C55-C59Two Knights Defence
C56Centre-Play Variation
C57Knight Attack
C58-C59Two Knights Gambit
C60-C99Ruy Lopez (Spanish Opening)
C61Bird's Defence
C62(Old) Steinitz Defence
C63Schliemann Defence (Jaenisch Gambit)
C64Cordel Defence
C65-C67Berlin Defence
C66incl. Chigorin Variation
C67incl. Rio de Janerio Variation
C68-C69incl. Exchange Variation
C70incl. Caro Variation and Classical Defence Deferred
C71-C76Steinitz Defence Deferred
C73Exchange Variation
C74incl. Siesta Variation
C76Bronstein Variation
C77-C99Morphy Defence (main line)
C77incl. Wormald Attack
C78incl. Archangel Variation
C79Russian Defence (Steinitz Variation)
C80-C83Open Morphy Defence
C80Bernstein System
C81Howell Attack
C83Classical (main Open line)
C84-C99Closed Morphy Defence
C84incl. Centre Attack
C85Delayed Exchange Deferred (DERLD)
C86Worrall Attack
C89incl. Marshall Counter
C91incl. Boboljubow Variation
C92incl. Lenzerheide (Zaitsev) Variation
C93Smyslov Variation
C94-C95Breyer Variation
C96-C99Chigorin Defence
C98Rubinstein Variation
ECO code
D00-D05Queen's Pawn Games
D01incl. Richter-Veresov Attack
D02incl. Krause Var.
D03Tartakower Variation
D04-D05Colle System
D06-D69Queen's Gambit
D06Irregular Defs. incl. Marshall Defence
D07Chigorin Defence
D08-D09Albin Counter Gambit
D10-D19Slav Defence
D13-D14Exchange System
D15incl. Slav/Geller Gambits
D16-D19Alapin Variation
D17Open Slav or Czech System
D19Euwe System
D20-D29Queen's Gambit Accepted
D21Borsenko-Furman System
D24Showalter Variation
D26-D29Classical Variation
D30-D69Queen's Gambit Declined
D31incl. Semi-Slav Defence
D32-D34Tarrasch System
D34Rubinstein System
D35-D36Exchange System
D37incl. 5.Bf4 System - the Modern Harrwitz
D38-D39Ragozin Variation
D40-D42Semi-Tarrasch Defence
D43-D49Semi-Slav Defence
D44Anti-Meran System
D46-D49incl. Meran Variation
D50-D69Pillsbury Development
D51incl. Manhattan Defence
D52incl. Cambridge Springs Defence
D54incl. Lasker System
D55-D59Neo-Orthodox Defence
D55Anti-Tartakower System
D56-D57incl. Lasker Variation
D58-D59Tartakower System
D60-D69Orthodox Defence
D61-D62Rubinstein Variation
D64-D69Classical Variation
D67-D69Capablanca System
D71-D79Neo-Grunfeld Defence
D75Kemeri Variation
D76Orthodox Fianchetto Variation
D78-D79Slav Variation
D80-D99Grunfeld Defence
D82-D84Brinckmann Attack
D85-D89incl. Exchange Variation
D85incl. Modern Exchange Variation
D87-89Spassky (Botvinnik Exchange) Variation
D90-D99Three Knights Variation
D93Bg3 System
D94-D95Quiet (Closed) Variation
D96-D99Russian Variation
D97Ragozin Variation (Classical)
D98-D99Smyslov Variation
E00-E59Indian Defences
E00-E09Catalan Opening
E02-E05Open Variation
E06-E09Closed Variation
E10-E19Anti-Nimzo Indian System
E10incl. Blumenfeld Counter Gambit
E11Bogo-Indian Defence
E12-E19Queen's Indian Defence
E12-E13"3Kts" Variation
E14"Attack" Variation
E15-E19Accelerated Fianchetto Variation
E17-E19Classical Variation
E20-E59Nimzo-Indian Defence
E22-E23Spielmann Variation
E24-E29Saemisch Variation
E30-E31Leningrad Variation
E32-E39Classical Variation
E33Milner-Barry Defence
E34-E37Noa Variation
E39Pirc Variation
E40-E59Rubinstein System
E41incl. Huebner Variation
E43-E45Nimzowitsch Variation
E46-E59Rubinstein Variation (main line)
E50-E59Modern Variation
E51-E59Ragozin Variation
E60-E99King's Indian Defence
E62-E69Delayed Fianchetto Variation
E63Carlsbad Variation
E64-E66Yugoslav System
E67-E69Classical System
E71Makogonov Variation
E72Deferred Fianchetto Variation
E73-E75incl. Averbach Variation
E76-E79Four Pawns Attack
E80-E89Saemisch Variation
E81incl. Byrne System
E82Double Fianchetto Variation
E83incl. Ruban Variation
E84Panno Variation
E85-E89Orthodox Variation
E87-E89Closed Orthodox
E90-E99Classical Variation
E92incl. Exchange Variation
E93Petrosian System
E95-E96Old Main Line
E97-E99Aronin-Taimanov Variation

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