Based on a set of empirical rules, the number of "losing tricks" held in each of the partnership's hands is estimated and their sum deducted from either 24 (the result is the number of tricks the partnership can expect to take when playing in their established suit, assuming normal suit distributions and assuming required finesses work about half the time) or 18 (the result is the bidding level the partnership can expect to make their contract when playing in their established suit, assuming normal suit distributions and assuming required finesses work about half the time). F. Dudley Courtenay originally referred to the latter option as the “Rule of 18”.
Based on a set of empirical rules the number of "losing tricks" held in each of the partnership's hands is estimated and their sum deducted from either 24 (the result is the number of tricks the partnership can expect to take when playing in their established suit, assuming normal suit distributions and assuming required finesses work about half the time,) or 18 (the result is the bidding level the partnership can expect to make their contract when playing in their established suit, assuming normal suit distributions and assuming required finesses work about half the time). F. Dudley Courtenay originally referred to the latter option as the “Rule of 18”.