The doubling cube was introduced in the United States in the 1920s. The cube is a die with the numbers 2,4, 8,16, 32 and 64 on its faces. It is used to increase the number of points at stake in a backgammon game.
When a player thinks he has a significant lead in a game, he can offer to double the game value to win more points (or money). Either player may offer the first double of the game value by placing the doubling cube showing the number 2 on the top on the half of the board to his right. A double may be offered only when it's a player's turn and before he has rolled the dice.
The opponent either accepts, in which case the points scored are doubled, or declines, in which case the player offering the cube wins a point. A double may be accepted or declined. If accepted, the accepting player owns the cube and is the only player who can offer the next double. If declined, the doubled player resigns and the game ends.
Most players new to backgammon believe they should double with any advantage and refuse the cube if they are losing. However, this is not always the case
Three factors should be considered when deciding to offer or accept a double:
1) Position - if you have an advantage in position, for example having made three or four blocking points in a row and have all of your checkers are no longer on your opponent's home board .
2) Race - if you require fewer moves to bring your checkers home and bear them off than your opponent.
3) Threats - You are the white player and have made a number of rolls that make four or three points, including 5-4, 5-2, 4.3, 4-2, 3-2, 3-1 and 2-1 plus various doubles. In that case, white should double and black should pass.
An important factor to estimate when you offer or accept a cube is the percentage of gammons. A doubled gammon scores four points so even if it occurs only a quarter of the time, that is the equivalent to the point you could have given up. Double early if there is a good chance of winning a gammon.
