Rule of Eleven

When a defender leads Fourth Best, you can subtract the card led from eleven, and this difference is the number of cards higher than the card led in the three other hands:

Q86

K7543 JT9

West leads the 4, his fourth best. 11 - 4 = 7 so there are 7 cards better than the four in the other 3 hands. When dummy plays low and South plays the Ace, East can be sure his partner has the King, as he can see 6 of the 7 cards between his hand and dummy, and declarer has just played the 7th.

This rule can also work to declarer's advantage:

QT43

KJ95 62

A87

West leads the 5. 11 - 5 = 6 better in the other 3 hands. Dummy plays low and when West plays the 6, South knows the full layout of the suit.