Editing Backgammon (section)

=== Doubling cube ===

[[Image:Backgammon DoublingCube.jpg|120px|thumb|Doubling cube]]
To speed up match play and to provide an added dimension for strategy, a doubling cube is usually used. The doubling cube is not a die to be rolled, but rather a marker, with the numbers 2, 4, 8, 16, 32, and 64 inscribed on its sides to denote the current stake. At the start of each game, the doubling cube is placed on the midpoint of the bar with the number 64 showing; the cube is then said to be "centered, on 1". When the cube is still centered, either player may start their turn by proposing that the game be played for twice the current stakes. Their opponent must either accept ("take") the doubled stakes or resign ("drop") the game immediately.

Whenever a player accepts doubled stakes, the cube is placed on their side of the board with the corresponding power of two facing upward, to indicate that the right to redouble, which is to offer to continue doubling the stakes, belongs exclusively to that player.<ref name="robertie-winners"/><ref name="hoyle"/> If the opponent drops the doubled stakes, they lose the game at the current value of the doubling cube. For instance, if the cube showed the number 2 and a player wanted to redouble the stakes to put it at 4, the opponent choosing to drop the redouble would lose two, or twice the original stake.

There is no limit on the number of redoubles. 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.<ref name="bill_robertie-beavers">{{cite web|url=http://www.gammonvillage.com/backgammon/news/early_beavers.cfm|title=Backgammon Beavers|access-date=2007-10-17|last=Robertie|first=Bill|publisher=GammonVillage}}</ref>

A variant of the doubling cube "beaver" is the "raccoon". Players who doubled their opponent, seeing the opponent beaver the cube, may in turn then double the stakes once again ("raccoon") as part of that cube phase before any dice are rolled. The opponent retains the doubling cube. An example of a "raccoon" is the following: 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, while 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 the 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", named after [[Oswald Jacoby]], 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.<ref name="robertie-serious">{{cite book|last=Robertie|first=Bill|author-link=Bill Robertie|title=Backgammon for Serious Players|edition=second|year=2006|pages=19–22|publisher=Cardoza|isbn=978-0-940685-68-0}}</ref>

The "Crawford rule", named after [[John R. Crawford]], 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.<ref name="robertie-serious"/> It is possible for a Crawford game to never occur in a match.

If the Crawford rule is in effect, then another option is the "Holland rule", named after [[Tim Holland (backgammon)|Tim Holland]], 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.<ref>{{cite web|url=http://www.bkgm.com/gloss/lookup.cgi?pat=holland+rule|title=Backgammon Glossary/Holland Rule|publisher=Backgammon Galore!|access-date=2009-02-26|archive-url=https://web.archive.org/web/20160312074549/http://bkgm.com/gloss/lookup.cgi?pat=holland+rule|archive-date=2016-03-12|url-status=dead}}</ref>