A simple mathematical formula stating that, if the lead is fourth highest, the number of cards capable of beating it in the other three hands is equal to eleven minus the pip value of the card led.
For example:
If East judges that the seven is a fourth-highest lead, he can use the rule of eleven. Taking seven from eleven leaves four cards higher than the seven, all of which he can see between his hand and dummy. He can deduce that South has no card higher than the seven and hence that he can play low at trick one to leave West on lead to play a second round of the suit through dummy’s king at trick two.