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The doubling cube

The cube is what turns backgammon from a dice game into a game of judgement.

The doubling cube is a six-sided die marked 2, 4, 8, 16, 32 and 64. It does not get rolled. It records the current stake, and offering it is a claim: I think I am winning enough that you should either pay more or give up now.

How a double works

1. Before rolling on your turn, you may offer the cube.
2. Your opponent either takes — the stake doubles and they now own the cube — or drops, conceding immediately and losing the current stake.
3. Whoever took the cube is the only one who may offer the next double.

That last rule is the whole game. Doubling hands your opponent the right to double you back later, so the cube is not only worth what it is now — it is worth control of what happens next.

When should you take?

The classic answer is that you can take with about 25% winning chances. The arithmetic is simpler than it sounds. If you drop, you lose exactly one point every time. If you take, you win 2 when you win and lose 2 when you lose. Break-even is where those match, which is winning one game in four.

Two things move that number in practice. Gammons push it up — if a meaningful share of your losses will be gammons at double the stake, you need more than 25% to justify taking. Owning the cube pushes it down, because the right to double back is worth real equity.

When should you double?

Roughly, when you are clearly better but the game is not yet decided. Doubling too early wastes the cube's value; doubling too late means your opponent simply drops and you collect one point for a game you were winning outright.

The Crawford rule

In match play, when either player gets to within one point of winning the match, the very next game is played without the cube. That single game is the Crawford game. After it, the cube returns — which is why a trailing player will often double immediately in the game that follows.