Ethereal – a UCI-compliant chess engine operating under the alpha-beta framework, paired with a Neural Network for positional evaluations. Ethereal is inspired by a number of open source projects and aims to serve as both a high-end engine and reference for other authors. The project strives to keep the source and its ideas, however complex, clean and digestible.
With the introduction of NNUE, the cost of training networks and competing with top engines, while still making an original effort, has gone up significantly. As a result, Ethereal moved to being commercial with the release of NNUE, a move which was met by support from the community, in order to ensure I could continue to work on the project.
Ethereal, as well as the projects that support Ethereal like OpenBench and NNTrainer are licensed under the GPLv3. The GPLv3 gives you, the user, the right to have access to the source code of the engine, the right to redistribute the GPLv3’ed portions of the project, as well as the right to reuse the Ethereal source in any capacity so long as you continue to comply with the GPLv3’s license.