This part of ISO 6489 gives specifications for the clevis couplings used in the attachment to agricultural towing vehicles of towed, non-balanced trailers and implements equipped with a coupling ring 40 as specified in ISO 5692-2 and ISO 8755. The mounting of one of the three trailer coupling shapes, A, B or C, on an agricultural vehicle or trailer is at the manufacturer's or user's discretion. The full interchangeability of the three shapes is provided for.