A suit of 5 or 6 (if 6, exactly 3-card support for pd's suit) cards, missing at most one honor. For example: ♠AQJxx, ♥KJxx, ♦Ax, ♣xx, after pd's 1♥ opening. If pd has 2nd round control on ♣, ♥A (even ♥Q is missing), ♠K, a slam is almost certain. Even with a singleton, sometimes I prefer to show double fit than shortness. If double fit is confirmed, we can then use double-suit RKC.