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:
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 leads the 4. Assuming this is a fourth-best lead, how many spades are in declarer's hand which are higher?
Thus, declarer holds exactly one card higher than the 4 if partner's lead was fourth-best. The full suit distribution around the table:
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