Open source licenses
Last updated
ChessFree is built on open-source work by other people. This page records what we use, under what terms, and where to get the source.
Stockfish (GPL-3.0)
The computer opponent is Stockfish 18, compiled to WebAssembly. Stockfish is free software licensed under the GNU General Public License version 3.
- Upstream engine source: github.com/official-stockfish/Stockfish
- WebAssembly build source: github.com/nmrugg/stockfish.js
- The exact binary this site serves: stockfish.js and stockfish.wasm
Under the GPL you are free to use, study, modify and redistribute Stockfish, provided you pass on the same freedoms. Stockfish runs on this site as a separate program in its own worker, communicating over the standard UCI protocol.
chess.js (BSD-2-Clause)
Move generation, validation and game-state detection use chess.js by Jeff Hlywa, under the BSD 2-Clause license.
cm-chessboard (MIT)
The board interface uses cm-chessboard by Stefan Haack, under the MIT license.
Chess piece graphics (CC BY-SA 3.0)
The Staunton piece images originate from Wikimedia Commons, created by Wikipedia user Cburnett and contributors, licensed Creative Commons Attribution-ShareAlike 3.0 Unported. Modified copies of these images must be shared under the same license.
Reporting a licensing problem
If you believe we have failed to attribute something correctly, tell us at moribust@protonmail.com and we will correct it promptly.