A formal rule of thumb in the card game of bridge. When defending against a no-trump hand, the defender that opens is expected by convention to lead the fourth card from his or her longest and strongest suit. The rule of eleven states that the numerical value of that card subtracted from 11 is equal to the number of cards higher than the card led that are outside that opener's hand.
Example: Opener's longest suit is diamonds, in which he holds: 3 6 8 10 A. By convention he leads the six. At this point the "dummy" (the winner of the contract who did not originate it) puts his/her cards down on the table, and has: 2 9 J in diamonds. The opener's partner now knows that there are 11 - 6 = 5 diamond cards higher than 6 outside the opener's hand. Two are obviously in dummy, and (say) one is in her hand, so she now knows that there are only two left to worry about in declarer's hand, and can plan an attack to defeat the contract.
Of course, the declarer can make the same kind of calculation, but since the defense is opening by attacking what is likely to be the weakest suit in the contract it is more valuable as information to them.