Available wind-tunnel data on the low-speed horizontal-tail contribution to the static longitudinal stability of high-speed airplane configurations incorporating unswept and sweptback wings are reviewed and analyzed. The characteristics of the flow behind wings and wing-body combinations are described and related to the downwash at specific tail locations for unseparated flow conditions. The effects of variations of tail position, variations of wing plan form and airfoil section, trailing-edge flaps, stall-control devices, and ground interference on the air-flow characteristics and tail contribution are analyzed and demonstrated. The more favorable tail arrangements are emphasized and their application to specific configurations is illustrated. The analysis of the factors affecting the horizontal-tail contribution is, for the most part, descriptive; however, an attempt has been made to summarize certain data by empirical correlation or theoretical means in a form useful for design.