This part of ISO 14531 specifies the physical properties and mechanical-performance requirements for crosslinked polyethylene (PE-X) pipes intended for use in the supply of gaseous fuels. In addition, it lays down dimensional requirements and specifies some general material properties (including chemical resistance) together with a classification scheme for PE-X produced in the form of pipe. This part of ISO 14531, when used in conjunction with the other parts of ISO 14531, is applicable as the basis for the design, manufacture and installation of PE-X piping systems (PE-X pipes, PE-X fusion fittings and mechanical fittings) for the supply of category D and category E hydrocarbon-based fuels (see ISO 13623) at