Backgammon
Get all your pieces into your home and then bear them off first to win
Backgammon is a two-player board game played with counters and dice. It dates back nearly 5,000 years to the regions of Mesopotamia and Persia. The earliest record of backgammon itself dates to 17th-century England, being descended from the 16th-century game of Irish.
For an introduction to learning backgammon see https://ukbgf.com/wp-content/uploads/An-Introduction-to-Backgammon-September-2019-V2.1-final.pdf.
Rules
The object of the game is for each player to bring all their checkers into their home board, and then to bear them off the board. The first player to get all their checkers off the board is the winner.
Each player starts with 15 checkers setup as shown in the starting diagram above.
From the perspective of the player playing with the white pieces (player 1) their home is on the bottom right (currently points 1-6) and their pieces moves anti-clockwise around the board. Whereas the black home (player 2) is on the top right (currently points 19-24) and their pieces move in the other direction (clockwise).
The points shown on the board are for the active player - and will swap each turn, such that a players checkers move from point 24 to point 1. This helps within keeping the notation consistent.
Pieces can be captured if they are landed upon by the opponent, however a piece cannot move onto a square if that square in occupied by 2 or more pieces of the opponent.
A captured piece must be placed back into the opponent's home (with the dice roll limiting where it can land 1-6 for the home spots) before that player can move a piece or take a piece off the board.
It is possible for a player to have no legal actions available once they have rolled the dice, in this situation they must just end the turn.
There is a special rule of 'gin' that states that "If the player whose time has run out is certain to win the match, no matter what is rolled in the remainder of the game, including an already rolled legal throw, the player is still declared the winner of the match."
Platform Gameplay
There is a preference which a user can set ("Play Forced Actions") to automatically play moves and end their turn if their is no choice to be made. This is a common feature on other Backgammon websites.
A turn in Backgammon is made up three phases, rolling the dice, moving pieces, and confirming. The rolling of dice happens automatically at the start of a turn, immediately giving the player the chance to make moves. A player may freely undo their moves back until the dice roll. This is achieved by clicking the 'UNDO' button on the game board. Undos are similar to takebacks, but undos do not require permission from your opponent and are allowed in tournament play. Due to the ability to undo parts of a turn there is a need to confirm the end of a turn. This is achieved by clicking on the dice or hitting the space-bar key.
Doubling Cube and Match Play
When creating a backgammon game in a Swiss tournament, you can choose to play with a doubling cube by selecting more than a 1pt match. The number of points a player needs to win a match is an odd number. 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 "centred, on 1". When the cube is still centred, either player may start their turn (except the very first 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 2 facing upward. This indicates the right to re-double, which is to offer to continue doubling the stakes, and belongs exclusively to that player. 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 re-double the stakes to put it at 4, the opponent choosing to drop the redouble would lose two, or twice the original stake.
A single game win is worth 1 point, if a game ends in a gammon (losing player hasn't borne off a checker) it is worth 2 points, and a backgammon (losing player hasn't borne off a checker and has one or more pieces on the bar and/or in the winner's home table) is worth 3 points.
Therefore one game of backgammon could be worth a lot of points, for example winning with gammon and a doubling cube worth 8 points would result in a 16 point win.
Crawford Rule
On PlayStrategy, we use the "Crawford rule", named after John Crawford, and it 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". The doubling cube is shown on the game board but it is unavailable and transparent. After the Crawford game, normal use of the doubling cube resumes. It is possible for a Crawford game to never occur in a match.
Limitations
- PlayStrategy requires a cap on the total 'plies' a game can have. This is set to 1000, which is an unlikely number to reach if both players are playing to win, but it is theoretically possible to hit even if this is true. If a game reaches 1000 plies, it will be automatically ended and marked as a draw. This is true for all variants, although is more notable in Backgammon, which has no official rules to permit a draw. It is the intention to look into alternatives for Backgammon so as to never have a situation where a draw can occur.
For context, a ply is any action within the game, and for Chess the terms ply/move/turn are all equivalent. In PlayStrategy's implementation of Backgammon, a normal turn consists of 4 plies (Rolling the dice, moving a piece with the first dice, moving a piece with the second dice, and confirming the end of turn). However, the number of plies in a turn is flexible depending on if a double is rolled and if there are legal moves, and so can range from a minimum of 2 and maximum of 6.