This International Standard specifies the interchangeability requirements of the various elements and dimensions for the mechanical coupling of towing vehicles whose coupling is mounted forward and below to centre-axle trailers. It is applicable to road trains for the commercial transport of low-density, high-volume cargo of the greatest possible variety. It is also applicable to corresponding equipment in cases where the same vehicle makes up part of a road train having more than one trailer. It is not applicable to dedicated or special combinations such as coupling systems with an extensible drawbar; nor does it specify limitations on maximum gross mass and overall dimensions, which are generally laid down by legislative requirements. Vehicle combinations whose couplings form the subject of this International Standard (see Annex B for examples) are not interchangeable with those whose couplings are rear-mounted (see ISO 11406).