Backgammon. Backgammon is one of the oldest known board games. Its history can be traced back nearly 5,000 years to archeological discoveries in Mesopotamia. It is a two player game where each player has fifteen pieces which move between twenty-four triangles according to the roll of two dice. The objective of the game is to be first to bear off, i.e. move all fifteen checkers off the board. Backgammon is a member of the tables family, one of the oldest classes of board games. Backgammon involves a combination of strategy and luck. While the dice may determine the outcome of a single game, the better player will accumulate the better record over series of many games, somewhat like poker. With each roll of the dice, players must choose from numerous options for moving their checkers and anticipate possible counter-moves by the opponent. The optional use of a doubling cube allows players to raise the stakes during the game. Like chess, backgammon has been studied with great interest by computer scientists. Owing to this research, backgammon software has been developed that is capable of beating world-class human players. Backgammon is not controlled by a dominating authority yet the rules of play are agreed by the international tournaments. Backgammon playing pieces may be termed checkers, draughts, stones, men, counters, pawns, discs, pips, chips, or nips. The objective is for players to remove all their checkers from the board before their opponent can do the same. As the playing time for each individual game is short, it is often played in matches where victory is awarded to the first player to reach a certain number of points. Each side of the board has a track of 12 long triangles, called points. The points form a continuous track in the shape of a horseshoe, and are numbered from 1 to 24. In the most commonly used setup, each player begins with fifteen chips, two are placed on their 24-point, three on their 8-point, and five each on their 13-point and their 6-point. The two players move their chips in opposing directions, from the 24-point towards the 1-point. Points 1 through 6 are called the home board or inner board, and points 7 through 12 are called the outer board. The 7-point is referred to as the bar point, and the 13-point as the midpoint. To start the game, each player rolls one die, and the player with the higher number moves first using the numbers shown on both dice. If the players roll the same number, they must roll again. Both dice must land completely flat on the right-hand side of the gameboard. The players then take alternate turns, rolling two dice at the beginning of each turn. After rolling the dice, players must, if possible, move their checkers according to the number shown on each die. For example, if the player rolls a 6 and a 3, the player must move one checker six points forward, and another or the same checker three points forward. The same checker may be moved twice, as long as the two moves can be made separately and legally: six and then three, or three and then six. If a player rolls two of the same number, called doubles, that player must play each die twice. For example, a roll of 5-5 allows the player to make four moves of five spaces each. On any roll, a player must move according to the numbers on both dice if it is at all possible to do so. If one or both numbers do not allow a legal move, the player forfeits that portion of the roll and his or her turn ends. If moves can be made according to either one die or the other, but not both, the higher number must be used. If one die is unable to be moved, but such a move is made possible by the moving of the other die, that move is compulsory. In the course of a move, a checker may land on any point that is unoccupied or is occupied by one or more of the player's own checkers. It may also land on a point occupied by exactly one opposing checker, or blot. In this case, the blot has been hit, and is placed in the middle of the board on the bar that divides the two sides of the playing surface. A checker may never land on a point occupied by two or more opposing checkers; thus, no point is ever occupied by checkers from both players simultaneously. There is no limit to the number of checkers that can occupy a point at any given time. Checkers placed on the bar must re-enter the game through the opponent's home board before any other move can be made. A roll of 1 allows the checker to enter on the 24-point, a roll of 2 on the 23-point, and so forth, up to a roll of 6 allowing entry on the 19-point. Checkers may not enter on a point occupied by two or more opposing checkers.