1) There is a feature of the Rule of Fifteen and it is a mathematical calculation and/or device employed to assist the player to remember how many higher cards are outstanding in a suit whether the partnership agreement’s leads are Third, Fourth, or Fifth best of his/her long suit. The player simply substracts the pip or rank of the card led from the number 15 and uses that Rule or mathematical calculation. If the card led is Fifth Best, then substract five from fifteen and use the Rule of Ten. If the card led is Fourth Best, then substract four from fifteen and use the Rule of Eleven. If the card led is Third Best, then substract three from fifteen and use the Rule of Twelve.
2) The Rule of Fifteen is not a mathematical calculation, but rather a General Rule of Thumb. Not everything about the game of bridge falls within the realm of calculations and mathematics. Sometimes you just have to apply logic and reasoning based upon certain information and practiced guidelines.
Simply stated, the Rule of Fifteen allows the bridge player in the Fourth Seat, after three passes, to determine whether or not he should open the bidding.
The Rule of Fifteen states that the Fourth Hand should open the auction if the number of High Card Points and the number of SPADES equal 15 or more.
These points are also called Pearson Points. The logic and reasoning behind the Rule of Fifteen considers the proposition that the high card points are very likely evenly divided between the two partnerships. Therefore, Fourth Seat should open only with a Spade suit, thereby declaring that he does have the boss suit. Possessing the Spade suit almost assures the partnership against intervention from the opponents.
The Rule of Fifteen can be used effectively, but should be adhered to. If Fourth Seat has less than 15, after adding the high card points and the number of Spades, then Fourth Seat should pass.