How to read a code page
Every page on this site is built from one list, replayed move by move. This page defines each word and number used on the code pages, so the pages themselves can stay short.
The list
The opening names, ECO codes and move sequences come from lichess-org/chess-openings, retrieved on 2026-10-28. It holds 3,865 lines under all 500 codes from A00 to E99, with 3,177 distinct names. The list is dedicated to the public domain under CC0. Names, spelling and the code each line is filed under follow the list exactly. Where the list gives one name more than one line, each line is a different move order into a position of that name.
The list contains moves and names only. It says nothing about how often an opening is played, how it scores or whether it is sound, and neither does this site.
Reference line
A code usually holds several lines. The reference line is the shortest of them; when several are equally short, the one belonging to the opening with most lines in the code wins. The board at the top of a code page shows the position at the end of the reference line, with the last move’s two squares tinted.
Plies and moves
A ply is one move by one side; a full move is two plies. Moves are written in standard algebraic notation: 5.Nc3 is White’s fifth move, 5...a6 Black’s reply. The deepest line of a code is its longest one, counted in plies.
Named positions and the road
A named position is the position at the end of any line in the list. The road on a code page lists every ply of the reference line and marks each named position it passes, with the code that position belongs to. When a stop is marked “by another order”, the line reaches that position with its moves in a different sequence from the named line’s own. The moves after the line leaves the last named position of a different code are marked in red on the score sheet: they are the moves that turn the parent position into this code.
Branches, continuations and transpositions
Codes that leave the same position are codes whose reference lines last passed the same named position before going their own way. Where lines continue from here counts the lines filed under other codes that pass through the code’s reference position. Other move orders lists transpositions: a named position of this code reached inside a line of another code by a different sequence of moves, or the other way round.
The position facts
- Material counts pawns as 1, knights and bishops as 3, rooks as 5 and queens as 9. A side is “ahead on material” when its total is higher; this is arithmetic, not an assessment.
- Minor pieces developed counts a side’s knights and bishops that are not on their starting squares.
- Moves spent on pawns counts that side’s pawn moves in the line.
- Pawns on d4, e4, d5, e5 counts the side’s pawns on the four centre squares.
- Squares attacked in the other half is our space count: the number of squares in the opponent’s half of the board (ranks 5 to 8 for White, 1 to 4 for Black) that at least one of the side’s pieces or pawns attacks.
- Legal moves is the number of moves the side to move could play.
- A half-open file has no pawn of that side but at least one of the opponent’s; an open file has no pawns at all.
- A bishop is fianchettoed when it stands on b2 or g2 (b7 or g7 for Black) with its own pawn one square in front on b3 or g3 (b6 or g6).
- A position is a mirror image when every piece has an opposite-coloured twin on the same file, reflected across the middle of the board.
Comparisons
When a page says a code is the longest, fullest or most developed, it compares reference lines inside one of three groups, always the narrowest that applies: the codes whose reference line carries the same opening name (used only for the 20 names that head six or more codes), the 100 codes of the same volume, or all 500 codes. Each measure is compared in one group only on a page. Ties are named as ties.
Volumes
The letter of a code is its volume. The volume titles used here (A flank openings, B semi-open games, C open games and the French Defense, D closed and semi-closed games, E Indian defenses) are the customary headings of the five ECO volumes.
What this site does not do
It does not rank openings by strength, popularity or results, recommend what to play, or use game databases. Every count is computed from the list itself with the python-chess library at build time.