Domineering

'Stop-Gate' redirects here. For the waterway feature also called stop gates, see floodgate

Domineering (also called Stop-Gate or Crosscram) is a mathematical game played on a sheet of graph paper, with any set of designs traced out. For example, it can be played on a 6×6 square, a checkerboard, an entirely irregular polygon, or any combination thereof. Two players have a collection of dominoes which they place on the grid in turn, covering up squares. One player, Left, plays tiles vertically, while the other, Right, plays horizontally. As in most games in combinatorial game theory, the first player who cannot move loses.

Basic examples

Single box

Other than the empty game, where there is no grid, the simplest game is a single box.

In this game, clearly, neither player can move. Since it is a second-player win, it is therefore a zero game.

Horizontal rows

This game is a 2-by-1 grid. There is a convention of assigning the game a positive number when Left is winning and a negative one when Right is winning. In this case, Left has no moves, while Right can play a domino to cover the entire board, leaving nothing, which is clearly a zero game. Thus in surreal number notation, this game is {|0} = −1. This makes sense, as this grid is a 1-move advantage for Right.