The doubling cube
The doubling cube lets a player who thinks they are winning double the stakes. The other player either accepts and plays on for twice as much, or gives up the game at the current value. It turns backgammon into a game of judgement as well as moves.
The rules
- The cube starts in the middle at 1 (it shows 64, its highest face). Either player may make the first double.
- You can only double at the start of your own turn, before you roll. A checker on the bar doesn't stop you.
- Your opponent takes (plays on at double the stake and now owns the cube) or drops (loses now, at the value before the double).
- After a take, only the cube's owner may double again (redouble): 2 → 4 → 8, and so on.
- At the end, the result (1, 2 or 3 for a single, gammon or backgammon) is multiplied by the cube. A dropped double never counts as a gammon.
Take or drop: the 25% idea
Ignoring gammons, dropping costs you 1 point. Taking puts 2 points at stake. If you win the game a quarter of the time, taking costs 2×0.75 − 2×0.25 = 1 point on average, the same as dropping. So:
Take if you expect to win at least about 25% of the time from this position. Gammons, the match score and owning the cube move this number, but it is the right starting point.
The other side of the same idea: a double is strongest when your opponent's chances are close to that line, so they face a hard choice.
The Crawford rule (matches)
In a match, the first time a player gets to one point short of winning, the next game is played without the cube: the Crawford game. After it, doubling is allowed again. This stops the trailing player from doubling every game for free.
| After game | Score | Next game |
|---|---|---|
| 1 | 2–0 | Cube in play |
| 2 | 2–1 | Cube in play |
| 3 | 4–1 | Crawford game: no cube |
| 4 | 4–2 | Cube in play again (post-Crawford) |
Money-game extras
- Jacoby rule: gammons and backgammons count only as a single game unless the cube has been turned. Money play only, never in matches.
- Beaver: after being doubled, a player may immediately redouble and keep the cube. Money play only.
On TavlArena
The cube is optional in backgammon games with friends. Games against the bots are played without it, and Turkish-style tavla has no cube at all (modern tavla uses it). The cube stops at 64.
Questions people ask
When can you double in backgammon?
At the start of your turn, before rolling, if you own the cube or it is still in the middle.
What happens if I drop a double?
You lose the game at the cube's value before the double, never as a gammon.
What is the Crawford rule?
In a match, the game right after a player first reaches one point short of winning is played without the cube.