The doubling cube explained

What the cube is

The doubling cube is a large die marked 2, 4, 8, 16, 32 and 64. It shows the stakes of the game: how many points the winner scores, before gammons are counted. At the start it sits on the bar showing 64, which by convention means the stakes are 1.

How doubling works

At the start of your turn, before you roll, you may offer to double the stakes by turning the cube to the next number. Your opponent then has two choices. They can take, and the game carries on at twice the stakes, or they can pass, also called dropping, and lose the game straight away at the stakes before the double.

If the cube is on 2 and you are doubled to 4, passing loses 2 points and taking means playing on for 4.

Owning the cube

Whoever takes a double owns the cube, and only they can double next. That is a real advantage: if the game later turns in their favour, they can redouble, while their opponent can't double at all. While the cube is still in the middle, either player may double.

On OneGammon the cube sits next to the board on the side of the player who owns it, and you double with the Double button before you roll.

When to take: the 25% rule

Suppose the cube is on 1 and you are doubled. If you pass, you lose 1 point. If you take, you play for 2: you lose 2 or win 2. Taking is better as long as you win at least one game in four, because three losses of 2 and one win of 2 cost 4 points in total, the same as passing four times.

So the rule of thumb is to take if you will win at least 25% of the time. Two things move that line. Gammons make taking worse, because you can lose 4 points instead of 2. Owning the cube makes taking better, because you can redouble if things turn your way, so strong players take some positions at around 21% to 22%.

When to double

Double when you are a clear favourite and your opponent can still take. If you wait until your position is overwhelming, your opponent simply passes and you win only the current stakes. Doubling a moment before that point, when the take is still close, is usually best. Players call missing that moment losing your market.

In a pure race you can judge this from the pip count. A common guide: offer a double when you lead by about 8%, redouble at 9%, and your opponent should pass once your lead is beyond about 12%.

The cube in a match

In a match the score changes every cube decision. Near the end, a player who is far behind should double early, and a player who needs only a single game may not want the cube turned at all. When a player first gets within one point of winning, the next game is the Crawford game and is played without the cube. OneGammon applies that rule for you, and its computer makes every cube decision by the match score.