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The doubling cube: raising the stakes in backgammon
The doubling cube is the invention that turned backgammon from a dice game into a game of nerve. It lets you raise the stakes in the middle of a game, and it gives your opponent a hard choice.
What the cube is
The doubling cube is a die with the numbers 2, 4, 8, 16, 32 and 64 on its faces. It records the current stake of the game. At the start it sits in the middle of the board, owned by nobody, and the game is worth one point (the cube shows 64 face-up, but counts as 1). The cube was introduced in the 1920s by players in New York, and it transformed the game into the version played in tournaments today.
How it works
- At the start of your turn, before you roll, you may offer a double if the cube is in the middle or on your side.
- You turn the cube to 2 and offer it to your opponent.
- Your opponent either takes (accepts, and the game continues for double the stake) or drops (resigns the game and pays the current stake, 1 point).
- If they take, they own the cube and place it on their side. Only they may offer the next double (a redouble, to 4), and then the cube passes back to you.
There is no limit on redoubles in principle, although the cube itself stops at 64. Remember that gammons and backgammons are multiplied by the cube: a gammon with the cube on 4 is worth 8 points. Read the rules for scoring.
Why it matters
Ownership of the cube is an advantage. The player who owns it can wait and double at the best moment, while the other player cannot. Without the cube, a player who leads in the race can simply play safely. With it, the player who is losing can be pushed out of the game before the luck turns.
When to double, in simple terms
Experts use computer rollouts, but a few ideas work for everyone:
- Double when you are clearly ahead, but before the opponent would have to drop. If your opponent would certainly drop, you have waited too long: you win only a single point where a double might have won more. The best time to offer is when you are winning and the opponent still has a reasonable take.
- Double when the position is volatile. If one lucky roll could swing the game, doubling now makes the opponent play on at risk. A position where your good roll next turn would make your opponent drop is a position to double now.
- Pure races: a common guide is that a lead of about 8% in the pip count is a double, 9% a redouble, and 12% or more makes it a drop for the other side.
When to take and when to drop
The simplest rule is the 25% rule. If you drop, you lose 1 point for certain. If you take, you play on for 2. Taking is right when you have at least a one-in-four chance of winning (25%), because the extra risk is paid back by the games you win. Below 25% you should drop. The rule is approximate, since owning the cube is worth a little extra, so a take is sometimes right with slightly less.
Gammon chances change the maths. If you are likely to be gammoned, the real cost of taking is more than 2 points and you need a better position to take.
Match play: the Crawford rule
In a match to a fixed number of points, the player who trails could double every game once the leader is one point from winning, because they have nothing to lose. The Crawford rule (named after John R. Crawford) removes this: the first game after a player reaches one point short of the match score is played without the cube. After that game the cube may be used again.
Money play: the Jacoby rule
In unlimited (money) play, the Jacoby rule says a gammon or backgammon scores only a single game if the cube is still in the middle. This keeps strong players from refusing to double just to play for a gammon. It is not used in match play.
Try it
The best way to learn the cube is to use it. BackgammonTalk has the doubling cube built in, so you can offer a double during a game with a friend on a live call, and hear them think about the take. See play backgammon online.
Frequently asked questions
Can I double at any time?
Only at the start of your own turn, before you roll, and only if the cube is in the middle or already on your side.
What happens if I drop a double?
You lose the game immediately and pay the value of the cube before the double was offered, usually 1 point.
Does a gammon count more with the cube?
Yes. A gammon doubles the value on the cube and a backgammon triples it. A gammon with the cube at 4 is worth 8 points, unless the Jacoby rule applies.