Rule of 10-12
 |
In contract 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 respectively, a defender can determine how many cards are higher than the card partner has led. Ten is used if the lead is fifth best [1] and 12 is used if the lead is the third best.[2] The Rule of 11 applies when the lead is the 4th best.
When following the Rule of 10-12, the lead is the third card from a three or four-card suit and the fifth card from a five-card or longer 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 an interior sequence is called a coded 9 & 10 lead.
Sometimes opening leader may choose to lead the fourth best card from a four-card suit against a notrump contract if the third card is a spot card. The spot-card lead may give declarer a trick that he can't get if the fourth best is led.
| Q 6 5 4 | ||
| K 9 7 3 | W N↑ S↓ E | A 10 2 |
| J 8 |
If the opening lead is the 7, the suit will be blocked and defense is unable to set up the 3rd winner in the suit.
Variation[edit]
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.