BridgeHands

Rule of 11 - Assuming fourth-best leads, subtracting the value of the opening lead from 11 determines the number of outstanding cards above the rank of the suit led. 

	J 9 8 2
6 [x x x]		10 [x x x]
	A [x x x]

For instance, if the opening lead is the Spade 6, then the Rule of 11 is computed as:

11 - 6 = 5 outstanding cards above the the Spade 6

If the Dummy contains the Spade J 9 8 2 (three above the cards above the Spade 6), then opener's partner (East) and declarer (South) can deduce that the remaining players (East and South) have the 2 card remaining cards above the Spade 6 (obviously excluding the Dummy's J 9 8).  

After South plays the 2 from the dummy, let's say East plays the Spade 10 and declarer South wins with the SA.  Since East normally tries to win the trick with the lowest necessary winner (lowest equal first), West knows East does not have the Spade 7.  As noted above, after viewing the Dummy's three high cards, only two cards remained above the the Spade 6.  Now with the play of the Spade 10 and Spade A, all players know West has all outstanding cards above the Spade 6: with a remaining holding of the K Q 7 and possibly other cards beneath the fourth-best lead (5 4 3).

If the opening lead was a 2, the opener must have exactly 4 cards in the suit, since a holding of 5 or more cards would require the leader to begin with a card higher than the 2.  Similarly, if the leader begins with a 3 and dummy contains a 2 (or another player holds the lower card), we can again deduce the leader has exactly 4 cards.  Opening with a 3 with the Dummy exposing a 2 in the suit would lead to the same deduction - leader held 4 cards.

On the other hand, if West held the Spade 10 and initially led that card then something is amiss with the Rule of 11 in this situation.  Subtracting 11 from the 10 spot would leave only one honor card remaining above the Spade 10, and since we call all see the Spade Jack in the dummy, that would mean West held the Spade A K Q 10 [x x x].  But when holding a either a running sequence or broken sequence, the opening leader should begin with the top of sequence (or top of inner sequence with one top honor separated by single gap in the sequence).  So in that situation, West's lead must be an unambiguous "Top Of Nothing" indicating to partner that West is lacking other honors in the suit and is making a safe, passive lead (perhaps not wanting to lead a singleton trump, a suit with an unprotected Ace, away from a suit with a King or Queen when declarer is in a 5 level or slam contract).