Backgammon Doubling cube

To speed up match play and to provide an added dimension for strategy, a doubling cube may be put into play. A standard doubling cube is a six-sided die marked with the numbers 2, 4, 8, 16, 32, and 64. At the start of each game, the doubling cube is placed on the board with the number 64 showing; the cube is then said to be "centered on 1". When the cube is centered, and before rolling the dice on his turn, either player may propose that the game be played for twice the current stakes. His opponent must either accept ("take") the doubled stakes or resign ("drop") the game immediately. If the opponent takes, the cube, showing the doubled stake, is moved to his side of the board. Thereafter, the right to re-double belongs exclusively to the player who last accepted a double. Whenever a player accepts doubled stakes, the cube is placed with the corresponding power of two facing upward.[1][2]

The game is rarely redoubled beyond four times the original stake, but there is no limit on the number of doubles. Although 64 is the highest number depicted on the doubling cube, the stakes may rise to 128, 256, and so on. In money games, a player is often permitted to "beaver" when offered the cube, doubling the value of the game again, while retaining possession of the cube.

A variant of the doubling cube "beaver" is the "raccoon." The player who doubled his opponent, seeing him beaver the cube, may in turn then double the stakes once again ("raccoon") as part of that cube phase before any dice are rolled. His opponent retains the doubling cube. E.g. White doubles Black to 2 points, Black accepts then beavers the cube to 4 points; White, confident of a win, raccoons the cube to 8 points, whilst Black retains the cube. Such a move adds greatly to the risk of having to face the doubling cube coming back at 8 times its original value when first doubling the opponent (offered at 2 points, counter offered at 16 points) should the luck of the dice change.

Some players may opt to invoke The Murphy rule or the "automatic double rule." If both opponents roll the same opening number, the doubling cube is incremented on each occasion yet remains in the middle of the board, available to either player. The Murphy rule may be invoked with a maximum number of automatic doubles allowed and that limit is agreed to prior to a game or match commencing. When a player decides to double his opponent, the value is then a double of whatever face value is shown (e.g. if two automatic doubles have occurred putting the cube up to 4, the first in-game double will be for 8 points). The Murphy rule is not an official rule in backgammon and is rarely, if ever, seen in use at officially sanctioned tournaments.

The Jacoby rule allows gammons and backgammons to count for their respective double and triple values only if the cube has already been offered and accepted. This encourages a player with a large lead to double, possibly ending the game, rather than to play it to conclusion hoping for a gammon or backgammon. The Jacoby rule is widely used in money play but is not used in match play.

The Crawford rule is designed to make match play more equitable for the player in the lead. If a player is one point away from winning a match, that player's opponent will always want to double as early as possible in order to catch up. Whether the game is worth one point or two, the trailing player must win to continue the match. To balance the situation, the Crawford rule requires that when a player first reaches a score one point short of winning, neither player may use the doubling cube for the following game, called the Crawford game. After the Crawford game, normal use of the doubling cube resumes. The Crawford rule is routinely used in tournament match play.[4] It is possible for a Crawford game never to occur in a match.

If the Crawford rule is in effect, then another option is the Holland rule, which stipulates that after the Crawford game, a player cannot double until after at least two rolls have been played by each side. It was common in tournament play in the 1980s but is now rarely used.