Licenses

Last updated 10 Oct 2026 · Also available in Español.

Out of Book is built on free and open-source software. This page credits the parts with their own terms and links to their licenses. The full list of components and their license texts is in the third-party notices.

The chess engine: Stockfish

The analysis board runs Stockfish, the free chess engine, in your browser. We serve an unmodified WebAssembly build of it, Stockfish.js 19 (the "lite, single-threaded" build), which runs as a separate program next to the app.

Stockfish is licensed under the GNU General Public License, version 3 (GPL-3.0). You may copy, modify and share it under that license.

If you'd like a copy of the exact source code for the files we serve, write to support@outofbookchess.com and we'll send it.

The chess pieces

The pieces on the board are the set drawn by Colin M.L. Burnett ("Cburnett"), © the author, used under the 3-clause BSD license (opens in a new tab). The same set is also offered under the GPL, the GFDL and CC BY-SA 3.0 on Wikimedia Commons (opens in a new tab). They come with the react-chessboard (opens in a new tab) component, © Ryan Gregory, MIT license.

Opening names

Openings are named with the Lichess chess-openings (opens in a new tab) list, which its authors dedicated to the public domain (CC0). Thank you, Lichess.

Fonts

The fonts are served from our own server, under the SIL Open Font License 1.1 (opens in a new tab):

Everything else

The app is written with Next.js (opens in a new tab) and React (opens in a new tab), uses chess.js (opens in a new tab) for the rules of chess and ts-fsrs (opens in a new tab) for spaced repetition, and draws its icons with Lucide (opens in a new tab). These and the other libraries the app is built with are listed, with their copyright notices and license texts, in the third-party notices.

The sounds are synthesized in your browser by the app's own code.