Type H Tightlock couplers are a variety of Janney coupler, typically used on North American mainline passenger rail cars. They are designed with mechanical features which reduce slack in normal operation and prevent telescoping in derailments, yet remain compatible with other Janney types used by North American freight railroads. On a standard-gauge railway, the nominal mounting height for the coupler (rail top to coupler center) is 33 inches (838 mm), with a 34+1⁄2 ± 1 inch (876 ± 25 mm) maximum height on empty cars and 31+1⁄2 ± 1 inch (800 ± 25 mm) minimum height on loaded cars.