Chess, with a little help.

Royal Flush Chess uses mature open-source chess software. No remote engine service, accounts or paid API is involved.

• chess.js 1.4.0 by Jeff Hlywa and contributors provides legal moves and notation. BSD-2-Clause license and copyright notice.
• Stockfish.js 19.0.1, © 2026 Chess.com LLC, based on Stockfish by T. Romstad, M. Costalba, J. Kiiski, G. Linscott and contributors. Lite neural network by Chris Bao (sscg13). The unmodified lite single-threaded engine runs in a separate worker. GNU GPL version 3.
• Chessground 10.5.0 by the Lichess contributors renders and animates the board and handles pointer/touch interaction. GPL-3.0-or-later. Matching TypeScript source, original artwork and reproducible build instructions.
• Spatial pieces by Maurizio Monge. MIT license and copyright. Configured ivory/navy gradients and strokes; baked shadows/editor metadata removed. Board colors, framing and cues use original CSS.

Your freedom to study and modify

The engine is free software, provided without warranty. You may copy, modify and redistribute it under its license. Download the complete matching engine source, embedded network and build instructions here, from the same site as the engine binaries. Exact upstream commit: 0d5dd7944f88c817c07a26bc4fb1905d5f9c46d6. The included ARCADE-BUILD.txt describes rebuilding with Emscripten 6.0.9.

The new chess interface and integration modules are licensed GPL-3.0-or-later. Their readable source is served as rules/state adapter, UCI client, worker delivery loader, interface, piece reference adapter, board adapter, board theme, stylesheet and page source. Existing unrelated arcade and band material keeps its own ownership and license terms. Rules/engine pins and hashes. Board/art pins and hashes.

Practice, at your pace

Difficulty names describe bounded engine searches, not tested Elo ratings. There is no clock or competitive leaderboard. Hints and takebacks are counted in the result. Standard material draws are detected; exhaustive recognition of every blocked dead position is not claimed.

Rules reference: FIDE Laws of Chess, articles 5 and 9. Repetition and 50-move claims are optional; fivefold repetition and 75 moves are automatic, with checkmate taking precedence.