Principle of Restricted Choice

If one opponent plays a critical card in a suit, his partner is twice as likely to have the adjacent card in the same suit. This is because of the Principle of Restricted Choice (“PRC”) - a mathematical theory that was found to have relevance at the bridge table by Terence Reese. He first expounded the theory in his epic book “The Expert Game”, written in 1958 - an inspirational read for any ambitious player. 

North used the Blackwood convention (4NT) to ask for aces, and when his partner’s 5♦ response (one ace) revealed that all aces were present, he bid 5NT to ask for kings. South’s 6♠ response indicated possession of the three missing kings so North bid the Grand Slam.

West’s ♥10 lead ran to declarer’s ♥J and declarer cashed ♠K. East’s ♠J fell and declarer used PRC to deduce that West was now twice as likely to hold the adjacent card - ♠Q. Thus when he followed by leading ♠4 and West played ♠9, he crossed his fingers and inserted ♠10. East discarded a ♣ so he breathed a sigh of relief, cashed ♠A felling West’s ♠Q, and claimed his Grand Slam.

ANDREW’S TIP: When one opponent plays a critical card in a suit, play his partner to have the adjacent card in the same suit.