Playing a game or match

This chapter describes how to play a game, a match or a session of money games against GNU Backgammon, or a human opponent. It also shows you how to get hints and evaluations while playing.

Backgammon is a game for two players, played on a board consisting of twenty-four narrow triangles called points. The triangles alternate in color and are grouped into four quadrants of six triangles each. The quadrants are referred to as a player’s home board and outer board, and the opponent’s home board and outer board. The home and the outer boards are separated from each other by a ridge down the center of the board called.

Rules by Tom Keith

Backgammon is a game for two players, played on a board consisting of twenty-four narrow triangles called points. The triangles alternate in color and are grouped into four quadrants of six triangles each. The quadrants are referred to as a player’s home board and outer board, and the opponent’s home board and outer board. The home and the outer boards are separated from each other by a ridge down the center of the board called the bar.

Figure 1. A board with the checkers in their initial position. An alternate arrangement is the reverse of the one shown here, with the home board on the left and the outer board on the right.

The points are numbered for either player starting in that player’s home board. The outermost point is the twenty-four point, which is also the opponent’s one point. Each player has fifteen checkers of his own color. The initial arrangement of checkers is: two on each player’s twenty-four point, five on each player’s thirteen point, three on each player’s eight point, and five on each player’s six point.

Both players have their own pair of dice and a dice cup used for shaking. A doubling cube, with the numerals 2, 4, 8, 16, 32, and 64 on its faces, is used to keep track of the current stake of the game.

The object of the game is for a player to move all of his checkers into his own home board and then bear them off. The first player to bear off all of his checkers wins the game.

Figure 2. Direction of movement of White’s checkers. Red’s checkers move in the opposite direction.

To start the game, each player throws a single die. This determines both the player to go first and the numbers to be played. If equal numbers come up, then both players roll again until they roll different numbers. The player throwing the higher number now moves his checkers according to the numbers showing on both dice. After the first roll, the players throw two dice and alternate turns.

The roll of the dice indicates how many points, or pips, the player is to move his checkers. The checkers are always moved forward, to a lower-numbered point. The following rules apply:

• A checker may be moved only to an open point, one that is not occupied by two or more opposing checkers.
• The numbers on the two dice constitute separate moves. For example, if a player rolls 5 and 3, he may move one checker five spaces to an open point and another checker three spaces to an open point, or he may move the one checker a total of eight spaces to an open point, but only if the intermediate point (either three or five spaces from the starting point) is also open.

Figure 3. Two ways that White can play a roll of [5] [3] .

• A player who rolls doubles plays the numbers shown on the dice twice. A roll of 6 and 6 means that the player has four sixes to use, and he may move any combination of checkers he feels appropriate to complete this requirement.
• A player must use both numbers of a roll if this is legally possible (or all four numbers of a double). When only one number can be played, the player must play that number. Or if either number can be played but not both, the player must play the larger one. When neither number can be used, the player loses his turn. In the case of doubles, when all four numbers cannot be played, the player must play as many numbers as he can.

A point occupied by a single checker of either color is called a blot. If an opposing checker lands on a blot, the blot is hit and placed on the bar.

Any time a player has one or more checkers on the bar, his first obligation is to enter those checker(s) into the opposing home board. A checker is entered by moving it to an open point corresponding to one of the numbers on the rolled dice.

For example, if a player rolls 4 and 6, he may enter a checker onto either the opponent’s four point or six point, so long as the prospective point is not occupied by two or more of the opponent’s checkers.

Figure 4. If White rolls [6] [4] with a checker on the bar, he must enter the checker onto Red’s four point since Red’s six point is not open. If neither of the points is open, the player loses his turn. If a player is able to enter some but not all of his checkers, he must enter as many as he can and then forfeit the remainder of his turn.

After the last of a player’s checkers has been entered, any unused numbers on the dice must be played, by moving either the checker that was entered or a different checker.

Once a player has moved all of his fifteen checkers into his home board, he may commence bearing off. A player bears off a checker by rolling a number that corresponds to the point on which the checker resides, and then removing that checker from the board. Thus, rolling a 6 permits the player to remove a checker from the six point.

If there is no checker on the point indicated by the roll, the player must make a legal move using a checker on a higher-numbered point. If there are no checkers on higher-numbered points, the player is permitted (and required) to remove a checker from the highest point on which one of his checkers resides. A player is under no obligation to bear off if he can make an otherwise legal move.

Figure 5. White rolls [6] [4] and bears off two checkers.

A player must have all of his active checkers in his home board in order to bear off. If a checker is hit during the bear-off process, the player must bring that checker back to his home board before continuing to bear off. The first player to bear off all fifteen checkers wins the game.

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.

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.

The following optional rules are in widespread use.

1. Automatic doubles. If identical numbers are thrown on the first roll, the stakes are doubled. The doubling cube is turned to 2 and remains in the middle. Players usually agree to limit the number of automatic doubles to one per game.
2. Beavers. When a player is doubled, he may immediately redouble (beaver) while retaining possession of the cube. The original doubler has the option of accepting or refusing as with a normal double.
3. The Jacoby Rule. Gammons and backgammons count only as a single game if neither player has offered a double during the course of the game. This rule speeds up play by eliminating situations where a player avoids doubling so he can play on for a gammon.

When backgammon tournaments are held to determine an overall winner, the usual style of competition is match play. Competitors are paired off, and each pair plays a series of games to decide which player progresses to the next round of the tournament. This series of games is called a match.

Matches are played to a specified number of points. The first player to accumulate the required points wins the match. Points are awarded in the usual manner: one for a single game, two for a gammon, and three for a backgammon. The doubling cube is used, so the winner receives the value of the game multiplied by the final value of the doubling cube.

Matches are normally played using the Crawford rule. The Crawford rule states that if one player reaches a score one point short of the match, neither player may offer a double in the immediately following game. This one game with no doubling is called the Crawford game. If the Crawford game is won by the trailing player then the doubling cube becomes available in all subsequent games (and it’s most often in the best interests of the trailing player to double immediately in these games).

In this example, White and Black are playing a 5-point match. After three games White has 4 points, which is just one point short of what he needs. That triggers the Crawford rule which says there can be no doubling in next game, Game 4.

There is no bonus for winning more than the required number of points in match play. The sole goal is to win the match, and the size of the victory doesn’t matter.

Automatic doubles, beavers, and the Jacoby rule are not used in match play.

If GNU Backgammon is properly installed on your system, you can start it by simply typing gnubg at the command prompt in a terminal window. There is also some options and arguments to GNU Backgammon. If you want to start GNU Backgammon with only the command line interface, you can type gnubg –tty or gnubg -t. With this option, GNU Backgammon starts without the graphic user interface. There is also other options which can be added at the startup. All legal options passed to GNU Backgammon can be found in the listing below. There is also a short description of each option.

The builds of GNU Backgammon which can be downloaded from several places on the net, usually comes with a installation system. The installation system will create a launch menu item in your start menu. Usually the menu item can be found by clicking: Start→Programs→GNU Backgammon→GNU Backgammon for Windows. Clicking this start menu item will start GNU Backgammon.

GNU Backgammon also has a command line interface. This is usually supplied as a separate executable file on Microsoft Windows. You can start this version of GNU Backgammon by selecting: Start→Programs→GNU Backgammon→GNU Backgammon Command Line Interface.

Once you have installed both an X11R6 server and GNU Backgammon for MacOS X, you can run GNU Backgammon for MacOS X by:

1. Start your X11R6 server;
2. Once your X11R6 server is running, in one of its Terminal windows (by default, Apple’s X11 opens one such window at startup, and XDarwin opens three), do the following:
   1. If you have installed GNU Backgammon in your private Applications folder, type “cd Applications/gnubg” to get into GNU Backgammon for MacOS X’s folder, and then type “./gnubg” to run it;
   2. If you have installed GNU Backgammon in the global, top-level Applications folder, type “cd /Applications/gnubg” to get into GNU Backgammon for MacOS X’s folder, and type “./gnubg” to run it;

To start a new game, match or session, you should click in the menu: File→New, or you can click the New button in the toolbar. This will open a dialog box , where you can select if you want to start a new game, match or session. Use the radio buttons to select if you want to start a match, game or session. If you select a new match, you can select the match length by typing the match length into the Match length field.

There is also a shortcut toolbar at the top of the dialog. If you click on a button in this toolbar, you will start a new match of the specified length immediately. If you click on the button with the dollar sign, you will start a new moneygame session immediatly.

There is also an area of the New dialog box, where you can select player setting. If you want to start a match or game against a friend, you can simply select the Human vs. Human option.

You can also set the new game, match or session to be played with manual dice, if you check the option for Manual dice. If this is selected, GNU Backgammon will prompt for the dice roll for each roll.

GNU Backgammon also comes with a Tutor mode. If tutor mode is active, GNU Backgammon will analyse each move you do, and warn you each time you are about to do a mistake. More about tutor mode later in this chapter. Tutor mode can be activated if you check the option in the dialog.

By clicking on the Modify player settings… button, you will see the whole player setting dialog. This dialog will be explained later in this document.

The simples way to roll the dice is to click in the right board area between the board points. You can also select from the menu Game→Roll or the keyboard shortcut Ctrl-R.

Figure: Click in the red rectangle to roll the dice

When you have moved your checkers to make the desired move, you can complete your turn, and picking up the dice, by simply clicking on the dice.

One way to move the checkers is to click on a checker you want to move and then drag it, while holding the mouse button down, to the desired destignation point. If you have the option Show target help when dragging chequers enabaled, you will see the a green guiding at any legal target point for the checker you are dragging. You can find this option by selecting Settings→Options…→Display.

Move the mouse cursor over the checker you want to move. If you now click the left mouse button, the checker will move the pips of the left die. If you have already moved one checker, clicking on a new checker will move it according to the pips of the remaining die. If you click with the right mouse button on a checker, the checker will move according to the right die.

Using this method for moving the checkers can be really effective. Some users perfere showing the higher die to the left. You can make GNU Backgammon show the highest die to the left if you select in the menu Settings→Options→Dice and check the checkbox labeled Show higher die on left.

Before you complete your move by clicking the dice, its possible to undo your move, to get the position before any checkers have been moved. You can either click the Undo button in the toolbar, or you can select Edit→Undo form the menu.

There are some shortcuts for moving the checkers. You can make a new point by right clicking on the empty point that you want to make. As an example, if you roll 31 as the opening roll, and you want to play 8/5 6/5, you simply place the mouse cursor on the 5-point and right click. Making points this way also works if you hit a checker, and with doubles.

Another shortcut can be done in the bearoff phase of the game. By clicking in the bearoff tray, two checkers will be born off according to the dice roll. It will only work if you can bear off a checker with both dice.

You can also set GNU Backgammon to autoplay forced moves. This feature can be enabled by selecting Settings→Options…→Game, and then check the checkbox Play forced moves automatically.

You can offer a doubling to your opponent, simply by clicking on the cube. This will offer the cube immediatly. You can also click Game→Double from the menu.

You can accept a doubling from your opponent by simply clicking on the offered doubling cube at the board. If you want to decline the doubling, you can right click on the cube.

There are also three buttons in the toolbar for handling cube offers. These buttons are marked Accept, Decline and Beaver. The beaver button is only activated in money game sessions where beavers are allowed.

Figure. The toolbar doubling buttons. These buttons can be used when a cube is offered. Click Accept to accept a doubling, click Decline to reject a doubling and click Beaver to beaver the offered doubling.

There are also menu options from the drop down menu for all these options. Click Game→Take from the menu, to accept an offered doubling. Click Game→Drop from the menu, to decline an offered doubling. Click Game→Redouble from the menu, to beaver an offered doubling.

It’s also possible to resign during a game. To resign during play, it simply to press the Resign button in the toolbar. This is the button with the image of a white flag. When this button is pressed, a simple dialog box will appear where you can select if you want to resign normal, gammon or backgammon.

Resignation can also be selected from the menu system. Click Game→Resign to offer your resignation. The same simple dialog box will appear to select whenever you want to resign normal, gammon or backgammon.

When a resignation is offered a white square will appear on the board area. The square has a number 1, 2 or 3. A square with a 1, means that a single game resignation is offered. If it’s a square with a 2, a gammon resignation is offered. A square with a 3, means that a backgammon is offered. If you’re using a 3D board, the resignation will be a white flag instead of square.

To accept the resignation, simply click on the numbered square. If you want to decline the resignation, you can right click on the numbered square. You can also use the same toolbar buttons as for accepting or declining a doubling. There is also a menu option for accepting and declining resignations. Click Game→Agree to resignation from the menu, to accept an offered resignation. Click Game→Decline resignation from the menu, to decline an offered resignation.

If you during a game want to get a hint of the best move or the best cube action you can press the Hint button in the tool bar or the menu option Analyse→Hint. This will open a hint window.

This Hint window shows a list of all possible moves for the position and dice roll. The different moves are sorted by how GNU Backgammon ranks each move; there is one move on each line in the list. The rest of the numbers may look a bit complicated and cryptic but, once you learn what each number means, it isn’t really that bad.

• Rank This is simply the number that shows GNU Backgammon’s rank of the move. All moves evaluated at a given ply are ranked above those evaluated at a lesser ply. If you use the 0, 1, 2, etc, buttons below the hint list to re-evaluate one or more moves, the moves will be moved accordingly. Therefore, for accurate results when re-evaluating a move that you are interested in, it is usually best to re-evaluate all moves above it in the list. (And slipping with the mouse and re-evaluating, eg, a 3-ply move at 2-ply may make the move “disappear” down to the bottom of the list!)

• Type This is a description of the evaluation that was used to calculate the results. Cubeful means that it is taking into account the possibility that someone may double. Occasionally you may see several moves with the equity of -1.000, (See Equity below), despite significantly different winning percentiles. This is because it thinks that if any of these moves are played then the opponent will double and you should pass. The n-ply is the depth to which GNU Backgammon analysed the move.

• Win This is the fractions of games that GNU Backgammon thinks this move will win. This number includes gammons and backgammons wins.
• W g This is the fractions of games that are expected to win a gammon. It includes backgammons wins as well.
• W bg This is the fractions of games that are expected to win a backgammon.

• Lose This is the fraction of losses expected by GNU Backgammon. Notice that this number is 1.0 - Wins. It includes gammon losses and backgammon losses.
• L g This is fraction of games lost with gammon for the listed move. It includes backgammon losses as well.
• L bg This is the fraction of games expected to lose a backgammon with the listed move.

• Equity This is the overall evaluation of the position by the program, after considering the different win/loss percentiles, the cube position, and the match score. A 1.000 would mean that you are expected to win a point, a 0.000 would mean that it is roughly equal, and a -1.000 that you are expected to lose a point.
• Diff. This is the difference in equity, perceived in comparison to the top ranked move.

• Move This is the move being evaluated. In the case of a red line, it is the move that was played.

If you’re thinking about a cube decision while playing you can press the same Hint button in the toolbar, as you pressed when you wanted checker play hint. The difference is that the dice has not been rolled yet, and a cube turn must be legal. The hint window will again appear, but this time it will look a bit different and it will show a hint of the correct cube decision.

The first lines is the window dialog shows the evaluation depth, and the cubeless equity with the evaluated probabilities. This equity is compensated for the match score. The cubeless equity for a money game, where the the calculation of equity is not compensated for match score, is also reported.

Next follow three lines with cubeful equities. In figure above, the cubeful equity for the player on roll for not doubling at this turn, is +0.123. The equity for a double and the opponent passing is (of course) +1.000, since the player then will win one point. (The number is normalised to cubevalue of one, so even if it is a redouble to 4 or 8 or higher, the cubeful equity for double/pass is still +1.000 for the doubler.) The third number in figure above is the cubeful equity for the player doubling and the opponent accepting the double. The different option will be listed in order with the best option highest. The number right to the listed equities is the differences from the cube decision considered best.

The last line states the best considered cube action. In the figure above, GNU Backgammon considers the best cube action to be No double, take. When percentage number right to the proper cube action, is a number which indicates the bluff potential. It is the necessary percentage chance you think you can bluff your opponent into passing the cube to make the double theoretically correct. This number is only available when the position is considered No double, take or Too good to double, pass.

As you may see in both figures above, there is a set of buttons below the move list or cube analysis. Here follows a short description of what each of the buttons does:

• Eval This button will evaluate the selected moves, or the cube decision with the current evaluation setting.

• … This will open the evaluation setting dialog. You can then modify the evaluation setting. More about evaluation settings in the next chapter.

• 0 1 2 3 4 buttons Pressing one of these buttons will evaluate the selected moves or cube decision at cubeful 0-ply or cubeful 1-ply or cubeful 2-ply and so on.

• Rollout Pressing this button will start a rollout1) of the selected moves or cube decision.

• … Pressing this button will open the rollout settings dialog. This dialog will be further described later in this manual.

• MWC Abbreviation for Match Winning Chance. If this button is pressed the values in the hint window will be shown as the chance to win the whole match. If this button is unpressed or there is a money game that’s played, the values in the hint window is reported as Equity. Note: The equity in a match is actually a recalculation from match winning chance to equity. This recalculated number is often refered to as EMG, Equivalent to Money Game. The EMG number is comparable to the equity in a money game, and are therefore labeled ‘Equity’.

• Show Clicking on this button will show the board as it would be after the selected move is made. The button is only available in the chequer play hint window.

• Move Clicking this button will make the move selected in the list. It’s also possible to make a move directly from the list by double clicking on the move. The button is also available only in the chequer play hint window.

• Copy Pressing this button will copy the text of all selected moves to clipboard.

• Temp. Map Pressing this button will show Sho Sengoku’s temperature map. This map is further described later in the document. When this window remembers its size and position it will really become useful.

One of the greatest tools for learning is Tutor Mode. Tutor mode can be activated by clicking Settings→Options… and then click on the Tutor flag. Activate Tutor Mode by checking the Tutor mode check box.

In Tutor Mode, GNU Backgammon will analyze your moves and/or cube decisions and compare them with its choices. You set the threshold for its alerts, either Doubtful, Bad, or Very Bad. For example, if you set it for Bad then it will only warn you when you make a bad mistake. It will then allow you to re-examine your choice, go right ahead with it, or provide a ‘hint’ - essentially, showing you its analysis. The error tolerance for each of the three thresholds may be set in the Analysis settings pane.

The tutor can give warnings on both cube decisions and checker play. If you want the tutor to only warn on cube decisions, you can uncheck the checkbox for the Checker play. Likewise, if you want it to only warn on checker play decisions, you can uncheck the checkbox for the Cube decisions.

In the above figure, you can see a warning dialog from the tutor mode. If you press the button labeled Play anyway, the move you made will be kept and the game will continue. If you press the Rethink button, the dialog will close and the checkers that you moved will be returned to their original positions. You can then rethink the position and try an alternative move. Keep doing this until the Tutor accepts your move as being good enough. Alternatively you can press the Hint button to show the hint window with its list of possible moves and their evaluations. Finally, if you press the End Tutor Mode button, the dialog will close and turn Tutor Mode off.

A similar warning window will also appear for poor cube handling.

The time control feature is not fully implemented with the user interface yet. Hopefully this will be improved in the future.