The purpose of this complement is to describe in an illustrated manner the development of the method for measuring interfacial tension by stretching a liquid interfacial film attached to a platinum ring. This method is described in the UNE 55-708 standard. Systems in which the aqueous phase is the most dense and cases in which the aqueous phase is the least dense are considered separately. This complement can only be applied when the determination of interfacial tension is carried out following the method that uses a platinum ring described in the UNE 55-708 standard.