After a 4^{th} best lead, the partner of the opening leader can use the Rule of 11 to place cards in declarer's hand. The rule works as follows:
 Subtract the denomination of the 4^{th} best lead from 11.
This tells you how many cards higher than the led card are in the other 3 hands (declarer, dummy, and the leader's partner)  Subtract the number of cards dummy has which are higher than the opening lead
 Subtract the number of cards you hold which are higher than the opening lead
 This gives you the number of cards which declarer has which are higher than your partner's 4th best lead
The Rule of 11 is just subtraction
11  
  the denomination of the 4^{th} best lead 
  dummy’s cards higher than the lead 
  your cards higher than the lead 
declarer’s cards higher than the lead 
Here is an example of using the Rule of 11.
Example #1  ♠ A T 2 ♥ A J T 6 ♦ Q 7 ♣ Q 9 6 5 

3NT by South ♦5 lead 
♠ 9 4 ♥ Q 9 4 3 ♦ K T 8 2 ♣ 7 2 
East uses the Rule of 11:
 11 – 5 = 6
North, East, and South have a total of 6 diamonds higher than the 5  North has 2 diamonds higher than the 5
 East has 3 diamonds higher than the 5
 11  5 – 2 – 3 = 1
 South has 1 diamond higher than the 5
Declarer can use the Rule of 11 to determine how many cards the partner of the leader has which are higher than the 4th best lead.
The Rule of 11 for Declarer
11  
  the denomination of the 4^{th} best lead 
  dummy’s cards higher than the lead 
  declarer's cards higher than the lead 
cards held by the opening leader’s partner which are higher than the lead 
Here is an example of declarer using the Rule of 11 to help with a decision at trick one.
Example #2  ♠ J T 2 ♥ J T 8 6 ♦ A J 2 ♣ 7 6 4 

1NT by South ♦7 lead 
?  
♠ K Q 9 ♥ A Q 7 ♦ 4 3 ♣ Q J T 9 8 
What should declarer play from dummy (North) on the ♦7 lead?
Declarer uses the Rule of 11 to determine how many diamonds above the 7 East holds.
 11 – 7 = 4
North, East, and South have 4 diamonds higher than the 7  North has 2 diamonds higher than the 7
 South has none
 11  7  2  0 = 2
East has 2 diamonds higher than the 7
If declarer calls for the ♦2 from dummy and East has a holding like ♦K 9 6, East will use the Rule of 11 to determine declarer has no cards higher the 7
11 – 7 – 2 – 2 = 0
So East has no reason to play their King, 3rd hand high. East knows the ♦7 lead will win the trick.
Declarer recognizes there is no upside to playing the ♦2 from dummy against an East that uses the Rule of 11. East will not go wrong.
Declarer should call for the ♦Jack hoping that West led the 7 from a holding like ♦ K Q 8 7 5. That is declarer’s only chance to win 2 diamond tricks against good defenders.
Tip: Get in the habit of using the Rule of 11 before playing to the 1^{st} trick
Here is a 57 second video on the Rule of 11.