Rule of 10-12
|This article relies on references to primary sources. (December 2008)|
In bridge, the Rule of 10-12 is applied when the opening lead is the third or the fifth best from the defender's suit. By subtracting the rank of the card led from 10 or 12, a defender can figure out how many cards are higher than the card partner has led. 10 is used if the lead is fifth best  and 12 is used if lead is the third best. Rule of 11 applies when lead is the 4th best.
When following the Rule of 10-12 lead is the third card from 3 or 4 card suit and the fifth card from 5+ cards suit. Leading the third best is also applied to interior sequences (K J 10 x x) which is less ambiguous than the top of interior sequence. The third best lead from interior sequence is called coded 9 & 10 leads.
Sometimes opening leader may choose to lead the fourth best card from 4 card suit against no trump contract if third card is a spot card. The spot card lead may give declarer a trick that she can't get if the fourth best is led.
|Q 6 5 4|
|K 9 73||W N↑ S↓ E||A 10 2|
If opening lead is 7 suit will be blocked and defense is unable to set up the 3rd winner in the suit.
Possible variation that helps giving count from 5+ cards suit is leading the third best from even number of cards and the lowest from odd number.