The Backgammon Doubling Cube
The doubling cube is a large die marked 2, 4, 8, 16, 32 and 64. It does not move checkers. Instead it keeps track of how many points the current game is worth. Introduced in New York clubs in the 1920s, it turned backgammon from a game of moving checkers into a game of judgment as well, and today it is part of every serious match.
Starting position of the cube
At the start of each game the cube sits in the middle of the board, usually on the bar, with 64 facing up. In that position it means the game is worth 1 point and neither player owns the cube. The 64 is simply the face that marks "not yet doubled".
Offering a double
Before rolling the dice on his turn, a player who feels his position is strong may offer a double. The opponent then has two choices:
- Take (accept): the game continues at twice its current value. The player who took now owns the cube and places it on his side of the board.
- Pass (also called a drop or refusal): the player concedes the game at its value before the double, and a new game begins.
Ownership and redoubles
Only the player who owns the cube may offer the next double, called a redouble. If the game turns in his favor, he can redouble to 4. His opponent again decides to take or pass, and if he takes, the cube goes back to him at 4. A centered cube can be used by either player; an owned cube belongs to one side only. This back-and-forth is fair: the player who accepted a double gets the exclusive right to the next one, which is valuable if the game swings.
Gammons and backgammons are multiplied by the cube. A gammon with the cube on 4 is worth 8 points.
The cube in match play
In a match, players compete to reach an agreed number of points, such as 5, 7 or 11. Each game's result, multiplied by the cube, is added to the winner's score. The match score strongly affects cube decisions: a player who needs only 2 points to win may welcome a double that his opponent would rather avoid, and a player far behind may need to accept bold doubles to catch up.
The Crawford rule is standard in match play. When one player first reaches one point short of winning the match, the next game, the Crawford game, is played without the cube. After that game the cube returns. In these post-Crawford games the trailing player should usually double at the first opportunity, since the leader needs only one game anyway.
Outside matches, some players use the Jacoby rule, under which gammons and backgammons count only if the cube has been turned. It is not used in match play.
When to take: the 25% take point
Suppose you are doubled and the cube would go to 2. If you pass, you lose 1 point. If you take, you play for 2. Imagine four identical games. Passing all four loses 4 points. Taking all four and winning one of them gives +2 − 6 = −4, the same result. So, ignoring gammons and the value of owning the cube, you can take if you win at least 25% of the time. Gammon chances push the real figure up; owning the cube pushes it down a little. Match score can move it a long way in either direction, which is why serious players study match equity tables.
When to double
Doubling too early hands your opponent the cube cheaply; doubling too late lets him pass a game he would have lost. The best time is usually just before your position becomes strong enough for a pass, when a good roll would make the game too strong for your opponent to take. Our backgammon tips include more practical cube advice, and the odds on the probability page help you estimate your chances. To see where the cube fits in the overall plan, return to basic strategy.