This International Standard lays down the dimensional characteristics necessary for mounting and 

interchangeability of the fifth wheel mounted on a mounting plate (standard fifth wheel, Clause 4) or directly on the frame (direct-mounted fifth wheel, Clause 5) of towing vehicles for semi-trailers. It applies to the fifth wheels intended to hitch on semi-trailers equipped with a: 

— 50 coupling pin as defined in ISO 337; 

— 90 coupling pin as defined in ISO 4086. 

Dimensions not specified are left to the discretion of the component manufacturer. 

Test conditions and strength requirements to be met by 50 and 90 fifth wheel coupling are specified in ISO 8717.