The Jacoby Rule

The Jacoby rule is named after Oswald Jacoby, the American games expert who was a champion in both bridge and backgammon. The rule is short: gammons and backgammons count only if the doubling cube has been turned during the game, that is, if one player has offered a double and the other has accepted it. If the game ends with the cube still in the middle, a gammon or backgammon scores only as a single game.

Why the rule exists

The Jacoby rule is used in money play, sometimes called unlimited play, where each game is scored on its own and there is no match target. Its purpose is to keep games moving.

Without the rule, a player with a crushing position has a reason not to double. If he doubled, his opponent would simply pass, and he would collect only one point. By playing on with the cube in the middle, he can hope to win a gammon worth two points or a backgammon worth three, while his opponent has no way to get out of the game. These games can drag on for a long time.

With the Jacoby rule, playing on without doubling earns nothing extra. The strong player either doubles while his opponent still has a take, or doubles when he is far ahead, and the opponent usually passes and loses a single game. Either way the game ends sooner or is played at a higher value with both sides having agreed to it.

How scoring works

When a game ends, either by a normal finish or by a pass, the result is the starting value multiplied by the cube value, and then by 2 for a gammon or 3 for a backgammon, if those count. For example, if one game is worth one unit, the cube is on 2 and the winner gets a gammon, the result is 1 x 2 x 2 = 4 units. If the same gammon had been won with the cube never turned, under the Jacoby rule it would be worth just 1 unit.

Effect on strategy

• Doubling early becomes more attractive for the stronger side, because a centered cube caps the gain at one point.
• The player who is behind can be a little more relaxed about gammon danger while the cube is still in the middle.
• Once the cube has been turned, gammons are fully alive again, so the normal rules of playing for gammon and saving the gammon apply.

Not used in match play

The Jacoby rule is never used in matches, where the goal is to reach a set number of points. In a match, a gammon is always worth twice the cube value and a backgammon three times, whether or not anyone has doubled. Matches have their own special cube rule instead, the Crawford rule. For the basics of doubling, see our article on the doubling cube.