The rule of Eleven 

The rule of eleven is a valuable rule that can be used by either the defense or equally by declarer. Always check your partner's opening lead using the "Rule of Eleven." which states that the player subtracts the number of the first card lead from the number 11, and then the result is the number of cards higher contained in the hands of the partner of the opening leader and the declarer and the dummy. This information is useful not only to the declarer, but also to the partner of the leader, who can apply the same calculation. Use this only for the opening lead, not to any other leads when leading to the second trick or any trick afterwards.

Calculate

Find out how many cards higher that the card lead:
1.) Count how many cards in your own hand in the suit which are higher than the opening lead
2.) Count how many cards in Dummy which are higher than the opening lead
3.) Add 2.) and 3.) together
4.) Subtract the value of the card led from eleven.
5.) The answer gives you the count of cards which are higher than the opening lead outstanding.

Example

In the hands below: The diamond 7 is led by the opening lead the third to play defender subtracts 7 from 11. (11-7 = 4)
The answer is 4. From this the defense can work out that after seeing the diamond 9 and King in their own hand, and the Jack of diamonds in Dummy, there is only one card higher than the opening lead outstanding. You have a very clear picture of the distribution of the high cards in the diamond suit after only one card has been played.