The Backgammon Doubling Cube

1. What is the doubling cube?

The doubling cube is a large die marked with the numbers 2, 4, 8, 16, 32 and 64. It is not an ordinary die that you roll — it is a stakes multiplier.

At the start of the game, the cube sits in the middle of the board with the “64” face showing (by convention, this means it is centred and either player may use it). The game is initially played for “1 point”. Once the cube comes into play, the stakes are multiplied by the value shown on the cube.

2. How it is used

The rules of the doubling cube are simple, but the strategic implications run deep:

1. A player may offer a double only at the start of their turn, before rolling the dice
2. The opponent may take: the game continues at double the stakes, and the opponent now owns the cube
3. The opponent may pass (drop): they concede the game and lose the current stake
4. Only the owner of the cube may offer a redouble. If you have taken a cube at 2, you alone can offer to turn it to 4
5. The faces of the cube stop at 64, but in theory the stakes have no limit; in practice, the cube rarely goes beyond 8

3. When should you double?

Offering the cube too early (when your advantage is slight) is a mistake. Too late, and your opponent will pass in a position that might have won you a gammon. The ideal doubling window lies between these two extremes.

Good situations for doubling

• A lead in the race: you are roughly 8–10% ahead in the pip count in a pure race (for example 8 to 10 pips at 100)
• Pressure from the bar: your opponent has one or two checkers on the bar and your home board is partly closed
• A strong prime: you hold a five- or six-point prime with opposing checkers trapped behind it
• A blitz under way: you are attacking aggressively and your opponent has not yet managed to regroup

Do NOT double if...

• You have exposed blots that could be hit
• The position is uncertain and could quickly swing the other way
• You are playing a back game — the cube is more useful to you in your own hands
• Your opponent will obviously pass (you are “too good”) — you would throw away your chance of a gammon

4. When to take or pass

The decision to take or pass is one of the most important in backgammon. The basic rule:

• Take if you have at least a 25% chance of winning the game
• Pass if your chance of winning is below 25%

By passing, you lose exactly 1 point (the stake before the double). By taking with a 25% chance, you might be tempted to write: 0.25 × (−2) + 0.75 × (+2) = −0.5 + 1.5 = +1.0... no, hold on.

Let us do the sum properly. With the cube at 2: taking with a 25% chance of winning = 0.25 × 2 − 0.75 × 2 = 0.5 − 1.5 = −1.0. Passing = −1.0. At 25% the two are exactly equal — that is the break-even point.

5. The 25% rule

The 25% rule (also known as the take point or take/drop threshold) follows directly from the calculation above. If you have more than a 25% chance of winning after taking the cube, you should take. If you have less, pass.

Adjusting for gammons

The 25% rule holds in a pure race. In more complex positions you must also take potential gammons and backgammons into account. If you risk being gammoned (losing 2 points instead of 1), your take point rises towards 30–35%.

6. Beavers and raccoons

In some informal playing circles (not in official tournaments), two optional rules are added:

• Beaver: when your opponent doubles you, you may take AND immediately redouble while keeping the cube on your side. You are signalling that you believe you have the advantage despite their double.
• Raccoon: in response to a beaver, the original doubler may immediately redouble a second time.

These rules are fun in friendly games but do not apply in the official tournaments of the World Backgammon Association.

7. The Crawford rule

In a match played to several points (for example, first to 9 points), the Crawford rule states that when one player is 1 point away from winning the match, the doubling cube may not be used in that game (the “Crawford game”). After that game, the cube becomes available again.

This rule keeps the game balanced: without it, a player 1 point from victory would automatically be doubled on the first move, which would make the strategy trivial.

8. Gammons and the cube

The interplay between gammons and the cube is crucial. A gammon is worth 2 points (or 2 × the cube value if the cube is in play). A backgammon is worth 3 points (or 3 × the cube value). Here is how this affects your decisions:

• When you have good gammon chances, double to the current value × 2 before it is too late
• When you risk losing a gammon, be more demanding about taking a double: with the cube at 2, that gammon would cost you 4 points
• Match equity (the value of your position within a match) changes completely when a gammon is at stake

9. The psychological side

The doubling cube is also a psychological weapon. Some players use it to “intimidate”, doubling in ambiguous positions where the opponent may go wrong out of anxiety. Others use it to buy time or disrupt the rhythm of the game.

The best protection: learn to calculate your winning chances objectively and ignore the pressure. If your analysis says “take”, take. Fear is the worst possible adviser when the cube is turned.