Grant Acedrex is a medieval chess variant dating back to the time of King Alfonso X of Castile. It appears in the Libro de los juegos of 1283.
The following rules are from the reconstruction of Jean-Louis Cazaux and Sonja Musser.
The king and rook move as they do in modern chess. Their Betza notation are thus K and R respectively.
The aanca (a beautiful and fearsome bird, very similar to a roc, sometimes translated incorrectly as gryphon) moves one square diagonally (like a ferz), before optionally continuing orthogonally outward any number of squares. Its Betza notation is t[FR].
The rhinoceros moves like a modern knight, before continuing diagonally outward any number of squares. (In H. J. R. Murray's translation, its first move is a non-capturing knight move, after which it acts as a bishop for the rest of the game.) Its Betza notation is t[NB].
The lion moves like a threeleaper or a camel, and can jump (a combined (3,0)-leaper and (3,1)-leaper). (In Murray's translation, it can only move as a threeleaper.) Its Betza notation is HL.
The giraffe moves like a zebra, a (3,2)-leaper. (In Murray's translation, it is a (4,1)-leaper.) In Betza notation is J.
The crocodile moves like the modern bishop. Its Betza notation is B.
The pawn moves like the modern pawn, but cannot make an initial double step or capture en passant. Its Betza notation is mfWcfF. When it reaches the other end of the board (the twelfth rank for White, or the first rank for Black), it promotes to the piece that was originally there: the exception is that a pawn promoting on the g-file becomes an aanca. For example, a pawn promoting on the a-file would become a rook.
Castling does not exist in Grant Acedrex. On its first move, however, a king may make a diagonal or orthogonal leap of two squares (Betza notation AD) in addition to its normal moves.
The game could be sped up by using octahedral dice to dictate which piece could move that turn, ranking them in the order: king (8), aanca (7), rhinoceros (6), rook (5), lion (4), crocodile (3), giraffe (2), and pawn (1).
The multi-variant Chess engines Fairy-Max and Postduif can play Grant Acedrex under the XBoard or WinBoard user interface.
