The Rule of Eleven is usually considered as providing useful information for the defenders. If partner’s opening lead against a notrump contract is fourth highest, we can subtract the card from eleven to determine the number of higher-ranking cards in the other three hands. Seeing how many higher cards are in dummy, and knowing how many higher cards we hold, we can determine how many higher-ranking cards are held by declarer. For example:
Suppose partner leads the 6 against 3NT, a low heart is played from dummy, we play the Q, and declarer wins the K. What information do we have from the Rule of Eleven?
Partner’s heart suit is established! Subtracting partner’s 6 from eleven leaves five higher-ranking cards
in the other three hands. There are two in dummy, the 9 and 7. We have two, the Q and 10.
We’ve seen one from declarer, the K. That means declarer has no hearts remaining higher than the
6. As soon as we gain the lead, we want to return a heart so partner can take the established winners.