Fairy-Max is a free and open source chess engine which can play orthodox chess as well as chess variants. Among its features is the ability of users to define and use their own custom variant chess pieces for use in games. Fairy-Max was derived from (also developed by H.G. Muller), one of the smallest programs to play complete FIDE chess. Therefore, Fairy-Max versioning started with version number 4.8, the version of micro-Max used.