Played on a 5x5 board. Three arrangements have been proposed, and the game opens by asking which one you want to play; the choice counts as the first move and is remembered for the next game.
The 1969 original, by Martin Gardner. Each player has 5 Pawns, 1 Rook, 1 Knight, 1 Bishop, 1 Queen and 1 King, and the two back ranks are in the same file order, so the Kings face each other on the e-file.
The same pieces, with Black's back rank mirrored rather than translated.
Jeff Mallett's asymmetric arrangement, where the two armies are not the same:
Castling: yes, in all three.
"En passant" capture: yes.
Double move for Pawns (start): yes when possible.