The Doubling Cube

The doubling cube lets players raise the value of a game while it is being played. It was introduced in New York backgammon clubs in the 1920s, and it turned backgammon into a much deeper game. Knowing when to double, when to accept and when to give up is a skill of its own, and good cube handling wins more points over time than almost any checker-play technique.

What the cube looks like

The cube is a large die with the numbers 2, 4, 8, 16, 32 and 64 on its faces. At the start of a game it sits in the middle of the board, usually with 64 showing, which simply means the game is worth one point and nobody owns the cube yet. There is no upper limit on doubling in money play, so a game could in theory go to 128, 256 and beyond, but in practice cubes above 4 or 8 are rare.

How a double 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.
Your opponent then has a choice. He can take (accept), and the game continues at twice its previous value. Or he can pass (also called dropping), and he loses the game at its current value.
The player who takes the double now owns the cube. It is placed on his side of the board, and only he can make the next double, called a redouble.
When the cube owner redoubles and the other player takes, ownership switches again. The cube keeps moving back and forth this way.

You can never double twice in a row: after you double, your opponent controls the cube until he decides to redouble.

When should you take?

The basic math is simple. If you pass, you lose one unit for sure. If you take, you play for two units. Ignoring gammons, taking is correct when you will win the game at least about 25% of the time. At exactly 25%, taking and passing cost the same on average: in four games you would lose three times two units and win once two units, a net loss of four units, the same as passing four times.

In real positions the take point moves. Owning the cube is worth something, because you can redouble later if the game turns, so you can sometimes take a little below 25%. On the other hand, if you are likely to lose a gammon, you need more than 25% winning chances to take. Match score also changes the numbers, which is why tournament players study match equity tables (see our probability tables).

Gammons, backgammons and the cube

At the end of a game the winner scores the value shown on the cube. A gammon doubles that, and a backgammon triples it. So if the cube is on 4 and the winner scores a backgammon, the game is worth 3 x 4 = 12 points.

Beavers and other optional rules

In money play, some players add a beaver. When you are doubled and believe you are actually the favorite, you may immediately redouble while keeping the cube on your side. A careless double to 2 can then become a game worth 4, with the cube still owned by the player who beavered. Some groups also allow a raccoon, an immediate redouble by the original doubler in response to a beaver. Beavers are not used in match play.

Two other rules are tied to the cube:

The Jacoby rule, used in money play, says that gammons and backgammons count only if the cube has been turned during the game.
The Crawford rule, used in match play, says that no doubling is allowed in the single game right after one player reaches one point short of winning the match.

The cube in matches

In a match, players race to a fixed number of points, for example 5, 7 or 11. Each game starts at one point, and the cube raises the number of points a game is worth. Because a match is won by reaching a target, the value of a double depends heavily on the score. A double that is a clear take at 0-0 can be a clear pass near the end of a match, and the opposite also happens.

The cube is what separates casual backgammon from the serious game. Start by learning the 25% rule, then watch how gammon chances, cube ownership and the score change it.