In matchpoint it is important to understand the posible gain or loss resulting from doubles. Here is a prime example: If your opponents get to a bad slam contract and they go down you will get a very good score. The field will be scoring 650, 660 or some such score and you will have a plus 100 for a very good score. If you double you might get an extra matchpoint. But if the slam is a good one the field will be scoring 980 or 990 and your opponents will score 1210 or 1230. By doubling you turned an average result into a bottom. Worse would be to double a minor suit slam when the field is bidding six no trumps or you double a slam you know will not make and your opponents run to a makeable slam. Now you have turned a top into a bottom. The double of a slam usually calls of an unusual lead from your partner (A Lightner Double).
The double of a five of a major contract the was bid to make (not a sacrifice bid) or a double of four of a minor is also bad matchpoint bridge. If you beat five hearts or five spades one trick you will get a very good score. If your opponents bid four diamonds andmake five or go down one you will usually get a good score.
Be careful of doubling part scores when the opponents are not vulnerable and you are vulnerable. It is very difficult to defeat a part score contract enough to compensate for a vulnerable game. Even of one trick set is not enough to compensate you for a lost two of a minor contract. Also be ready to double vulnerable opponents when your game might be in doubt. A doubled vulnerable two trick defeat is all that is necessary to compensate for a not vulnerable game.
