ECO code - Encyclopaedia of Chess Openings

ECO code

Definition

An ECO code (Encyclopaedia of Chess Openings code) is a standardized five-character label used to classify chess openings and their sub-variations. The system ranges from A00 to E99, where the leading letter (A–E) designates a broad family of openings and the two-digit number (00–99) pinpoints a specific line or cluster of related lines. ECO codes originate from the renowned Encyclopaedia of Chess Openings (Chess Informant) and are widely used by players, coaches, authors, and databases to organize and study opening theory.

How ECO codes are used in chess

• Database search and organization: Filter millions of games by ECO (e.g., “B90” for Sicilian Najdorf main lines).
• Opening repertoire building: Group your lines by code to map coverage and avoid theoretical gaps in your Opening repertoire and Home prep.
• Study and annotation: PGN files often include an [ECO "Code"] tag for quick reference; authors cite ECO in books and articles to locate lines in Book and Theory.
• Communication shorthand: Players and coaches say “I play C65–C99” to mean specific Ruy Lopez ranges, or “I’m switching to D37–D43” for Queen’s Gambit Declined lines.
• Preparation: Identify an opponent’s preferences (e.g., “they love E60–E99”) to tailor Prepared variation and anti-lines.

Structure of ECO codes

The ECO is divided into five letter groups, each spanning 100 numeric subdivisions:

• A00–A99: Flank and irregular openings (e.g., English Opening, Larsen, Dutch, and many offbeat A00 lines).
• B00–B99: Semi-open games (especially the Sicilian Defense family, plus Caro–Kann, Pirc/Modern, and others).
• C00–C99: Open games and the French Defense, including the Ruy Lopez and related e4 e5 systems.
• D00–D99: Closed and semi-closed games (Queen’s Gambit family, Slav/Semi-Slav, Grünfeld, Tarrasch).
• E00–E99: Indian Defenses (Queen’s Indian, Nimzo-Indian, King’s Indian, Benoni/Benko, Catalan crossover lines).

Important: ECO indexing follows positions, not just move orders. Transpositions can land you in the same ECO via different sequences.

Examples by ECO family

• A00: Irregular (includes Grob 1. g4, certain offbeat first moves; many “experimental” systems live here).
• A10–A39: English Opening; A40–A99 covers a wide flank spectrum (including Dutch Defense and rarities).
• B20–B99: Sicilian Defense, where B90–B99 is the Najdorf—one of the most theoretically dense territories in all of chess.
• C00–C19: French Defense; C20–C59: Open Games; C60–C99: Ruy Lopez (Spanish), including famous Closed lines.
• D30–D69: Queen’s Gambit Declined and variations; D70–D99: Grünfeld Defense and related complexes.
• E00–E59: Nimzo-/Queen’s Indian/Bogo-Indian/Catalan systems; E60–E99: King’s Indian Defense.

PGN and database examples

A typical game header includes an ECO tag such as [ECO "B90"]. In practice, you might study a Najdorf line like:

Example 1 — Najdorf Sicilian (ECO often B90):

Tag: [ECO "B90"]

Example 2 — Ruy Lopez, Closed, Breyer (commonly C95):

Tag: [ECO "C95"]

Strategic and historical significance

The ECO system, published by Chess Informant starting in the 1970s, brought a universal index to opening Theory across languages and publishers. It enabled precise cross-referencing for analysts, accelerated the spread of novelties (TN), and supported the rise of deep Home prep in professional chess. With the advent of Engine analysis and mega-databases, ECO codes remain a backbone for organizing research, finding Book move vs. offbeat choices, and evaluating “drawing lines” versus “fighting” options with better practical chances.

Famous games and their ECO codes

• Fischer vs. Spassky, World Championship 1972 (Game 6) — Ruy Lopez, Breyer: commonly C95. A model strategic masterpiece in a classic C9x Spanish structure.
• Kasparov vs. Anand, World Championship 1995 (Game 10) — Sicilian Najdorf: B90. A showcase of dynamic counterplay and deep preparation.
• Morphy vs. Duke Karl/Count Isouard, “Opera Game,” 1858 — Philidor Defense: C41. A historic miniature demonstrating rapid development and a direct king hunt.

Practical tips for using ECO codes

• Build a repertoire map: list your main lines by ECO (e.g., White: C54–C59 Giuoco Piano, Black: B12 Caro–Kann Advance, E70 KID Fianchetto) to ensure coverage.
• Prepare efficiently: before an OTB game, search an opponent’s last 50–100 games by ECO to see which branches they repeat.
• Track novelties: tag your files with ECO + key move number where your novelty (TN) appears to revisit and refresh lines.
• Mind transpositions: learn the typical pawn structures and piece placements so you recognize when a different move order lands in “your” ECO territory.
• Balance risk: some ECO ranges are notorious “drawing weapons,” others are swashbuckling. Choose codes that match your style and desired Practical chances.

Common pitfalls and notes

• ECO revisions: Databases sometimes update or fine-tune boundaries; a line’s code can shift as theory evolves.
• Transposition traps: The same middlegame can arise from different ECO groups; don’t over-focus on codes at the expense of understanding plans.
• Granularity varies: Some popular systems (e.g., Najdorf B90–B99) are subdivided very finely; niche systems may be grouped more broadly.

Related terms and helpful links

• Opening and Opening theory: How ECO fits into study plans.
• Book and Opening book: Using ECO to navigate literature.
• TN and Prepared variation: Finding and cataloging novelties within an ECO range.
• Engine and Computer move: Verifying lines and assessing Eval.
• Home prep and OTB: Bridging preparation and practical play.

Interesting facts and anecdotes

• Shorthand culture: Many professionals refer to entire repertoires by ECO ranges—“I play A20–A29 as White” is common in seconds’ notes.
• A00 magnet: A00 collects a menagerie of offbeat first moves, from 1. g4 to 1. Na3—handy for the experimenter or the ambusher.
• Najdorf density: The B90–B99 block is among the most theoretically loaded in chess; it’s where countless world championship novelties have appeared.
• PGN synergy: Even though the “Seven Tag Roster” doesn’t include ECO by default, most modern PGN files add [ECO "…"] because it’s so useful for indexing.
• Style signaling: Announcing “I study E60–E99” often signals a combative, dynamic player—King’s Indian lines are famed for initiative and king attacks.

Mini reference: popular ECO hotspots

• A20–A39: English Opening main lines.
• B50–B99: Sicilian labyrinth, including the Scheveningen, Najdorf (B90–B99), and more.
• C60–C99: Ruy Lopez galaxy, from Open Spanish to the Closed Breyer/Chigorin/Marshall complexes.
• D30–D69: Queen’s Gambit Declined and offshoots; D70–D99: Grünfeld universe.
• E60–E99: King’s Indian Defense in all its dynamic glory.

Bottom line

The ECO code is the lingua franca of opening classification. Mastering its structure helps you search, study, and communicate about openings with precision—supercharging your Home prep and sharpening decisions from move one.