In the 1920s, players in New York City introduced a new wrinkle to Backgammon in the gaming clubs of the Lower East Side. They called it the “doubling cube,” a six-sided counter with the numerals 2, 4, 8, 16, 32, and 64 on its faces.
This cube was initially used for keeping track of the stakes being wagered, but it soon became an even more important part of the game, forcing players “not only to select the best move in a given position, but also to estimate the probability of winning from that position, transforming Backgammon into the expected value-driven game played in the 20th and 21st centuries.”
The traditional game of Backgammon is played for an agreed stake per point and each game begins with a wager of one point. The player that bears off all 15 pieces first wins how ever much has been staked, but some special rules apply to the amount bet.
For example, if the losing player has not borne off any pieces, a “gammon” is scored by the winner, worth double the stakes. Or, if the loser has no pieces borne off and at least one runner still on the Bar or in the winner’s Home Board, “backgammon” is declared, costing the loser three times the amount staked. Another common rule, called “automatic doubles,” states that if identical numbers are thrown on the very first roll of the game, the stakes are automatically doubled.
Introduction of the doubling cube brought even higher stakes opportunities to the game.
During the course of play, a player may propose doubling the stakes. If the opponent refuses the offer, the game is forfeit and the declining player must pay whatever the current stakes are. If the opponent accepts the double, the doubling cube is turned to the numeral 2 and becomes the possession of the accepting player.
Only the owner of the cube may propose doubling the stakes, and the offer must be made only at the start of his/her turn, before rolling the dice. In many forms of the game, the doubling cube can be used by either player at the beginning of the game, in which case the cube rests on the Bar initially. In other versions of Backgammon, the cube starts out as the possession of whichever player lost the opening roll of the dice.
Subsequent doubles during the same game are referred to as “redoubles.” Should a player refuse a redouble, the game is lost and the cost is whatever number of points showed on the doubling cube before the redouble was offered. Upon acceptance of a redouble, the cube is turned to the number 4 or whatever amount represents twice the previous stake. The player who accepted becomes the owner of the cube and the game continues. The number of redoubles that can occur in a game has no limit, but if there are no doubles at all, the stake remains at one point.
At the end of the game, as long as the losing player has borne off at least one piece, the cost of the loss is the value showing on the doubling cube. Gammons and backgammons still apply, however, so the stakes may be two or three times the point value on the doubling cube if the loser is unable to bear off any pieces or gets a runner trapped.
To propose or accept a double, the player should firmly believe there is a substantial possibility of winning. Keep in mind that the opportunity to redouble follows any accepted offer. The game situation can change quite quickly when doubles are thrown or blots get bumped.
A few special rules that affect wagering have also been incorporated into the game. To speed up play, the “Jacoby Rule” requires that gammons and backgammons count only as a single point if neither player has offered a double during the course of the game. The “Crawford Rule” states that whenever a Player comes within one point of winning in match play, the following game is played without the doubling cube.
One other popular rule is called “Beavers.” A player who is doubled may immediately redouble (“beaver”) while retaining possession of the cube. That means the original player who doubled must either accept or forfeit, just as with a normal double.