Mountain Pass Chess uses all sixteen chess pieces per side, but they are used on a 5 square X 7 square board, with three pawns, one bishop, one knight and one rook held in reserve to be brought in on the first row as space is available. Chess rules apply with some exceptions: Castling is allowed but only on the King side; the placement of a reserve piece to a first row space constitutes a move, and an opponents piece can be captured if it happens to occupy that space; as in Backgammon, a doubling cube can be used for side wagers, but is not necessary for playing the game; From the box: MOUNTAIN PASS CHESS has been discribed as incorporating the most revolutionary change in the rules for regular chess since the change that allowed castling. The game provides the opportunity for new study in openings, middle game and end game pley. As stated on the cover of this box... THE ULTIMATE CHESS CHALLENGE."