The Rule of 11 is a mathematical corollary to fourth-best leads. It enables the third hand player to count how many cards declarer holds which are higher than the opening lead. The Rule works as follows:

1. Subtract the opening lead spot card from 11.

2. Also subtract the number of cards in dummy that are higher than the card led.

3. Finally, subtract the number of cards in your hand that are higher than the card led.

4. The final number equals how many higher cards declarer holds in the suit.

The Rule of 11 can be confusing, so it's easiest to demonstrate it with an example:

 Partner ♠ 4 Declarer Dummy ♠ ? ♠ 10 9 6 You ♠ K J 5

Partner leads the 4. Assuming this is a fourth-best lead, how many spades are in declarer's hand which are higher?

1. Partner's spot card is the 4, so 11 - 4 = 7.

2. Dummy contains three spades higher than the 4, so 7 - 3 = 4.

3. You hold three spades higher than the 4, so 4 - 3 = 1.

Thus, declarer holds exactly one card higher than the 4 if partner's lead was fourth-best. The full suit distribution around the table:

 Partner ♠ Q 8 7 4 Declarer Dummy ♠ A 3 2 ♠ 10 9 6 You ♠ K J 5