Built in 1956 for the MANIAC I computer at Los Alamos National Laboratory — the first chess-like game ever played by a computer. The board was shrunk to 6×6 and the bishops removed to fit the era's processing power.
A 6×6 board with a king, queen, 2 rooks, 2 knights, and 6 pawns per side — no bishops. Piece movement is identical to ordinary chess. The pawn's double-step, en passant, and castling are all disallowed. Pawns cannot promote to a bishop (there are none on the board to begin with).
Simplification: check and checkmate aren't enforced — capturing the king directly ends the game.