The Rule of Eleven

When you are absolutely sure that the lead card is the 4^th highest of the suit, you subtract the number of the card lead, from the number 11, the result is the number of HIGHER cards contained in the hands of the partner of the 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 mathematical calculation. This information can be useful in deciding to play which card, either from the hand of the partner of the leader, or the hand of the declarer or from dummy.  For example, card lead 7 Diamonds, subtract from 11 = 4 cards higher in the other 3 hands.  You know how many is in Dummy and your own and can then deduce how many in the other hand.  Against no trump contracts, because the lead is usually 4^th highest, this rule is one of the most useful.