Encyclopaedia of Chess Openings

The Encyclopaedia of Chess Openings, usually shortened to ECO, was created as a reference system for organizing the many ways a chess game can begin. An opening is the early phase, when players bring pieces out from their starting squares, contest space, and begin shaping the pawn structure, meaning the way the pawns are arranged. Chess Informant created the ECO classification and incorporated it into its publishing work.

The system divides openings into five broad groups, A through E, with one hundred numbered codes in each group. The full range therefore runs from A00 to E99. A code can classify a family of openings or variations without relying on an opening name that may differ between languages. This makes ECO especially useful in books, game databases, and software that need a consistent way to organize large numbers of games.

An ECO code is not a score and does not say whether an opening is good. It should not be treated as one single immutable move sequence either: a category can cover different branches and different move orders, meaning different sequences that reach the same opening family. The traditional five-volume encyclopaedia corresponds to the letters A, B, C, D, and E and develops the analysis within those categories.