The backgammon doubling cube: rules, and when to take
The doubling cube shows 2, 4, 8, 16, 32 and 64. It doesn’t move checkers: it sets how much the game is worth. It turns backgammon from a game of luck into a game of decisions.
The rules
- At the start the cube is in the middle, and either player may double.
- You double only on your turn, before you roll.
- Your opponent chooses: take, and the game is worth double and the cube goes to them, or pass, and they lose the current value at once.
- Only the player who owns the cube can double again. The cube passes from side to side, and each time the value doubles.
- A gammon or a backgammon multiplies the value on the cube: a gammon with the cube on 4 is worth 8.
When to take: the 25% rule
If you pass, you lose one point for sure. If you take, you play for 2: winning 25% of the time, you gain 2 in those games and lose 2 in the rest, which averages out to losing one point, the same as passing. So you take when your chance to win is at least about 25%.
- In practice you can take with a little less, around 22%, because you own the cube and can redouble if the game turns.
- When a gammon is possible you need more: losing a gammon costs double.
When to double
You double when the game is good enough for you that your opponent has to think, but before they can simply pass. For money that is roughly when your chance to win is 68% to 70%, and in a straight race when you lead the pip count by about 8%. Wait too long and your opponent just passes, and you win only one point.
Special rules
- Crawford: in a match, in the first game after a player gets one point away from winning, nobody may double.
- Jacoby: in money play, gammons and backgammons count only if the cube was turned.
- Beaver: a player who is doubled may immediately redouble and keep the cube. Used only in money play.
In many cafés people play with no cube at all. Here you choose: with the cube or without, and at the end the analysis shows every cube decision too.