Bridge is a game of probabilities. A player makes a particular bid or play because he thinks that, percentage-wise, it is the bid or play that will work out best.

He does not necessarily make these bids or plays because he is certain they will succeed, since if a genuine choice exists, it is possible the less-likely choice will prove to be the winner.

Take this deal where South is in six hearts and West leads a club. Declarer’s only possible losers are a spade and a diamond, so he must try to avoid losing one or the other.

There are two ways to proceed. After drawing trump, declarer can play the A-K and another spade, hoping the opposing spades are divided 3-3, in which case his diamond loser can be discarded on dummy’s fourth spade.

Alternatively, declarer can try to develop an extra diamond trick by taking two finesses in the suit. This approach succeeds if West has the king or queen of diamonds, or both, in which case South’s spade loser can eventually be discarded on one of dummy’s diamonds.

Any experienced player, whether familiar with the exact percentages or not, would choose the latter line of play. He would instinctively know that this offered by far the better chance of success.

In terms of actual percentages, the 3-3 division offers only a 36% chance of succeeding, while the probability of finding West with one or both diamond honors is about 75%. Consequently, the prospect of developing a 12th trick in diamonds rather than spades is more than twice as promising. Even so, it is important to remember that this is merely a percentage play, so that on a bad day, the inferior play might win while the superior play might lose.

Tomorrow: When to break a general rule.