Let's look at a typical backgammon match situation. You are behind, and you have been forced into a back game. The cube is at 2 and you have it. The match is up to 9 points and Black is leading 5-1. On the back game, Black, in a single game, would double you now if he owned the cube, to make you pay for the chance of pulling the game out of the fire. But here you should double him! If Black wins the game at all, he will probably win a gammon anyway, and with it the match. The last thing he wants is to have the stakes increased. It cannot help him to win a gammon at 4 rather than at 2. But it does very much help you to win a single game at 4 rather than 2. So you must double. It's odd, isn't it? In a single game he should double you if he could, and you should drop. Yet in a match at this score, in exactly the same position, you double him. He should accept, of course, but he won't be happy to 'get the cube' (i.e. be doubled).
Now think about another problem. You are leading 3-1 in a match up to 5 and the game swings in your favour. You have a definite chance of winning a gammon, but you could quite easily still lose the game. Normally, in a single game, you would double at this stage. Probably Black would drop, because he would be afraid of losing 4 points. But now Black could not care less whether he loses 2 points or 4. It does not matter to him in the least that there is a danger of losing a gammon if he accepts, since he will lose the match anyway if he loses a single game. Do you realize what Black is going to do, if he does accept? He will redouble you straight back to 4, whatever the position. The whole match is now going to depend on this game, irrespective of whether it's a gammon or a single game. So the right policy is not to double him, but to play on, going all out for a gammon. If he does offer you a double, you must accept but change your tactics. You have no interest in winning a gammon now because a single game will win the match for you. But you must still avoid losing a gammon, because that would cost you the match.
Example match situation
Suppose the score is 13-all in a match up to 15. Or it might be 3-all in a match up to 5. It makes no difference as long as you each need 2 points to win. What should be your tactics with doubling cube? Is it obvious to you immediately? If so, I will bet anything you like that you are wrong. It may look simple. In fact, it's quite complicated. As it's a score line which crops up very often, it's worth going into.
First, let's take the case where the Crawford rule is not being played.
- No Crawford rule. 13-13 in match to 15. If you are ahead and have a chance of a gammon, you must not double. Black will refuse if you do and will double you at once in the next game. Your only advantage will then be a free drop. But if you had not doubled him, you might have gammoned him and won the match on this game. If you are behind, you obviously don't usually double either. But if it's one of those games where you are probably going to lose a gammon if you don't hit him, but win if you do hit him, then you are the one who must double.
If you are ahead but there is no chance of a gammon, offer a double at once, however small your advantage. Black must refuse, so all you win is a simple game, and a free drop. But that is all you could win anyway, so you must take it immediately, rather than play on giving Black the chance of a lucky win.
- Crawford Rule being played. 13-13 in match to 15. If you have a very good chance of a gammon, don't double. Just as when the Crawford Rule is not played, try to win the match on this game. If you are behind, double if it's a game where you will either win the game or lose a gammon. You are going to lose the match anyway if you get gammoned, so make sure you win the match if you do happen to win the game.
If there is no question of a gammon, double whenever you would do so in a money game. But you must also double whenever you have the advantage, if there is a chance that Black may not be in a Position to accept next time. Remember that Black's right to redouble you is irrelevant, because if he has accepted your double the match depends on this game anyway.
It is implied in an otherwise discussion of this subject that Black should accept the double more readily at this score than in a single game: 'he is almost forced to take the double, since if he drops, he must win the next two games (barring gammons) to win the match.' But this is wrong. In a single game, as you know, he should take the double as long as his chance of winning is anything better than one in four. But here he needs a somewhat higher chance than that to justify taking the double. Here is the reasoning. It is true that, if he refuses, he has approximately one chance in four of winning two games running. But the added possibility (however cautiously you play) that he may win the first game with a gammon, gives him better than one chance in four of winning the match.
In Diagram, for instance, it is White's throw and he doubles. White has 26 chances in 36 to get off both men, and of the times where he fails to get them both off, Black wins 17 times out of 18 (whenever he does not throw 2-1). Black will win, on average, just over 26 times out of 100 if he accepts. For money he should accept (and in fact he will win whenever White fails to get off because White will reject the redouble), but at this point in a match he should refuse, because the chance of winning a gammon on the next game, in addition to the chance of winning two games running, gives him a better chance than 26 out of 100. Bear in mind, too, that the fact that White will have to be careful to avoid losing a gammon next game will lead him to alter his play and thus reduce his chance of winning the game. So actually Black will win two games in succession rather more than one time in four.
If you do have a chance of a gammon you should not let that put you off doubling, unless it is a very solid chance. It is still valuable to get the cube to 2, or to get Black to concede the game, if you can. But the kind of double you avoid giving is the very early one where a good part of the reason for doubling, in a money game, would be the chance of a gammon.
Diagram was reached by Black starting with a 6-3, coming all the way out with a back man and your throwing double 5, pointing on his blot on your one point, and making your three point. Black failed to come in. You would double in a single game because, although Black may easily win, you may gammon him. But at this score in a match play on for the gammon.
Be very careful about doubling when you have a good lead and are close to victory. Suppose you lead 13-11, playing up to 15. You should be much more inclined to play for a gammon than to double. The reason is that, if Black accepts, he will redouble you to 4 immediately and play this game for match. Winning 4 points rather than 2 won't help you at all. Losing 4 will cost you the match. This principle applies at earlier stages, too. Suppose you lead 11-7 in a match up to 15 and it is one of those games that could be a gammon either way. You need a very considerable advantage to justify you in doubling. Black will probably redouble to 4, if he accepts, and then if it is you who lose a gammon, you lose the match with it.