Chess openings: explorer with ECO codes and variations

ECO codes and the opening tree: how a position gets its name

ECO codes sort openings under five letters, from A00 to E99. The tool relies on lichess's open database of 3,865 named lines: it shows the name and code of the current position, or of the last named position along your path, then the moves that lead to further variations.

How the tool names a position

Each line of the lichess database pairs an ECO code, an English name and a sequence of moves. Those moves were replayed once with chess.js to store the resulting position (the FEN without move counters). After each move on the board, the tool looks the position up; if it is not there, it steps back one move at a time to the last named position, the method the database authors recommend. Matching positions rather than move orders catches transpositions.

Worked example

The database holds 3,865 lines and 500 codes from A00 to E99: 823 lines under A, 781 under B, 1,250 under C, 644 under D and 367 under E. From the starting position it lists 20 first moves; 1. e4 opens 2,032 lines and 1. d4 opens 1,333. After 1. e4 c5 (B20, Sicilian Defense), 20 White replies are listed and 2. Nf3 covers 302 lines. The orders 1. Nf3 d5 2. d4 and 1. d4 d5 2. Nf3 reach the same position, named D02 Queen's Pawn Game: Zukertort Variation.

Limits

Names are the database's own, in English. No game statistics are shown: the lichess games explorer requires an OAuth2 login and the tool calls no online service. For a position reached by a move order missing from the database, next moves only appear if that position is itself listed. Promotion is always to a queen.

Sources and dates: lichess-org/chess-openings (CC0); chess.js (BSD-2-Clause); lichess API, Opening Explorer (OAuth2); pieces by Colin M. L. Burnett (BSD) (checked 29/10/2026)

Frequently Asked Questions

What does an opening's ECO code mean?

It is a letter from A to E followed by two digits, from A00 to E99, grouping openings into families. The lichess database uses all 500 codes for 3,865 lines: the Sicilian Defense, 1. e4 c5, is B20 and the Ruy Lopez, 1. e4 e5 2. Nf3 Nc6 3. Bb5, is C60.

How do I find an opening's name from the moves played?

Play the moves on the board or pick them from the list of next moves. The tool shows the position's name and code; once you leave the database, it keeps the name of the last listed position and says after which move you left it.

What is a transposition in chess?

It is a different move order that reaches the same position. 1. Nf3 d5 2. d4 and 1. d4 d5 2. Nf3 lead to the same position, which the tool names D02 Queen's Pawn Game: Zukertort Variation, because it compares positions rather than move sequences.

Why doesn't the tool show win percentages?

Those figures come from game databases. The lichess games explorer requires an OAuth2 login and the tool calls no service: it loads the tree of 3,865 named lines once, with no game results.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.