It is surprisingly rare that an in-depth knowledge of probabilities is key to landing a contract (relief!). Even if you do know that the à priori chances of (say) a 4-1 split are 28%, those odds will probably be different when you come to broach the suit: the bidding and play to date will have altered those odds. All you really need to know about odds in practical play is:

A missing odd number of cards rate to split as evenly as possible (eg five missing cards rates to split 3-2). Therefore a finesse (50-50) is less likely than the most even split.

A missing even number of cards does not rate to split evenly (eg four missing cards do not rate to split 2-2 [this is because a 3-1 (or the less likely 4-0) split can occur in two ways: three of the left – one on the right; or one on the left – three on the right]). Therefore a finesse is more likely than the even split.

Which of these suits is most likely to generate an extra trick? Least likely?

However in practical play, a comparison of these odds is not normally necessary.

Say you are wide-open in spades and need one extra trick from the other three suits (as above). You would cash ♣ AKQ and ♦ AKQ in some order, finishing with a king. If either clubs have split 3-2 (quite likely – 68% if you’re curious) or diamonds have split 3-3 (quite unlikely – 36%), you have a long card. If neither cooperate, then the last resort is to lead to ♥ Q.

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