A doubling cube is an object used to record the current value of a game that has been doubled one or more times. It's shaped like a die and has the numbers 2, 4, 8, 16, 32 and 64 on its six faces. A player who wishes to double the value of the game picks up the doubling cube before he rolls his dice, and offers it to his opponent with the side labelled 2 facing up. If the opponent accepts the double, he "owns" the cube and places it on his side of the board. The owner of the cube can choose to double at any later time when it is his turn, before he rolls the dice. He would offer it to his opponent with the "4" facing up. The original doubler then owns the cube, and can double again on his turn, and so on.

Here is an explanation of the doubling cube and of gammon and backgammon from Backgammon Galore!

I forgot to mention in an earlier post that multipoint matches would require the system to recognize gammon and backgammon as well.


Doubling
Backgammon is played for an agreed stake per point. Each game starts at one point. During the course of the game, a player who feels he has a sufficient advantage may propose doubling the stakes. He may do this only at the start of his own turn and before he has rolled the dice.
A player who is offered a double may refuse, in which case he concedes the game and pays one point. Otherwise, he must accept the double and play on for the new higher stakes. A player who accepts a double becomes the owner of the cube and only he may make the next double.

Subsequent doubles in the same game are called redoubles. If a player refuses a redouble, he must pay the number of points that were at stake prior to the redouble. Otherwise, he becomes the new owner of the cube and the game continues at twice the previous stakes. There is no limit to the number of redoubles in a game.



Gammons and Backgammons
At the end of the game, if the losing player has borne off at least one checker, he loses only the value showing on the doubling cube (one point, if there have been no doubles). However, if the loser has not borne off any of his checkers, he is gammoned and loses twice the value of the doubling cube. Or, worse, if the loser has not borne off any of his checkers and still has a checker on the bar or in the winner's home board, he is backgammoned and loses three times the value of the doubling cube.