Using the Backgammon Doubling Cube
Many casual players have never touched the doubling cube, yet in tournament backgammon it is as important as the checkers. It was introduced in New York clubs in the 1920s and quickly became part of the game. The cube turns every game into a series of decisions: not only how to move, but also when the game is worth more and when it is time to give it up.
The cube itself
The doubling cube is a large die with the numbers 2, 4, 8, 16, 32 and 64 on its faces. At the start of each game in a match it sits in the middle, usually on the bar, at the value 1 (shown by the 64 face). At that moment nobody owns it and either player may use it.
How a double works
- Offering. A player who feels he has a good advantage may offer a double at the start of his turn, before he rolls the dice. He turns the cube to 2 and passes it to the opponent.
- Dropping. The opponent may refuse. The game ends at once and the doubler wins it at the current value, that is, before the double.
- Taking. The opponent may accept. The game continues, now worth twice as much, and the player who took becomes the owner of the cube. He keeps it on his side of the board.
- Redoubling. Only the owner may double next. If the game turns in his favour, he may redouble to 4 at the start of one of his turns. The other player then faces the same choice: drop and lose 2 points, or take, play for 4 and own the cube.
The cube can travel back and forth like this, and each pass doubles the value of the game. A gammon or backgammon multiplies the cube value as well: a gammon with the cube on 4 is worth 8 points.
The take point
When should you accept a double? The basic idea is the take point: the minimum share of games you need to win for a take to be better than a drop.
Take a simple case with no gammons and a match far from its end. If you drop, you lose 1 point for sure. If you take, you play for 2 points. Taking costs you nothing extra if you win at least one game in four: in four such games you would lose three times 2 points and win once 2 points, a net loss of 4, the same as dropping four times. So the classic take point is about 25%. Owning the cube after the take improves your chances a little more, because you can redouble later if the game swings.
In match play the take point moves with the score. Near the end of a match one game can decide everything, and gammons can be worth much more or much less than usual. Serious players use match equity tables to work out the take point for each score. Our article practice makes it perfect explains how to work on these skills.
The Crawford rule
In a match to N points, as soon as one player reaches N-1 points (one point short of winning), the next game is played without the cube. This is the Crawford game. Without the rule, the trailing player would double at the first opportunity, since he has nothing to lose by doing so. After the Crawford game the cube comes back into play for all remaining games, and the trailer will normally double at once.
An example in a 5-point match:
| Game | Score before the game (you : opponent) | Cube |
|---|---|---|
| 1 | 0 : 0 | Allowed |
| 2 | 0 : 2 | Allowed |
| 3 | 0 : 4 | Not allowed (Crawford game) |
| 4 | 1 : 4 | Allowed (post-Crawford) |
Other cube rules
- The Jacoby rule is used outside matches and does not apply in match play. Under it, gammons and backgammons count only if the cube has been turned during the game.
- The Holland rule was an older match rule: in post-Crawford games the trailer could double only after both sides had played two rolls. It is rarely used today.
Learning to use the cube
Good cube handling comes from experience. Note every cube decision you are unsure about, check it later with a program such as GNU Backgammon or eXtreme Gammon, and you will soon develop a feel for when to double and when to take. If you need a refresher on scoring, see the backgammon rules.