# Chess training content

## Verified tactical puzzles

The 54 tactical exercises in `js/chess/training-data.js` are a curated subset of the official Lichess puzzle database, obtained 2026-11-06:

- Original publisher: Lichess and its puzzle-generation contributors.
- Source and format documentation: https://database.lichess.org/#puzzles
- Dataset: https://database.lichess.org/lichess_db_puzzle.csv.zst
- License: **CC0 1.0 Universal** (https://creativecommons.org/publicdomain/zero/1.0/).
- The publisher explicitly allows downloading, modification, redistribution, commercial use, and publication without permission. Attribution is not required by CC0; it is provided here as provenance.
- Each exercise retains its original puzzle URL, original database rating, and original tactical themes in its data record. Ratings describe the source puzzle, not a user rating or an engine Elo estimate.
- Original database FENs precede an opponent lead-in. That lead-in was legally applied using chess.js before saving the learner-facing starting FEN. Remaining UCI moves were legally converted to SAN. The source solution is preserved exactly.
- Only a bounded prefix of the compressed database was fetched; the entire multi-million-puzzle dataset is not shipped.
- The database documents that solution player moves are decisive only moves, except that mate-in-one puzzles can have alternative mating moves. The trainer may accept an alternative move only if it is verified checkmate in a mate-in-one exercise.

All selected FENs and solution sequences are tested in `tests/chess-content.test.mjs`. Mate-category exercises must finish in checkmate. The tactical descriptions are original explanatory prose.

## Opening lines, lessons, and endgame positions

The 17 opening records, 23 lessons, instructional prose, and 14 endgame setups were authored for this project. Standard chess moves, opening names, and rules are factual material. There is no copied instructional artwork or third-party lesson prose. Chessboard diagrams are rendered interactively using the project’s licensed piece assets.

Lessons are instructional lines, not claims that every recommended move is the only winning move. Endgame technique exercises use active engine defense; they are not tablebase-certified win/draw assertions. Read the supplied goal and explanation when selecting an exercise.