This International Standard specifies the design lengths and size of threads for polyethylene (PE)/metal adaptor fittings and polypropylene (PP)/metal adoptor fittings for pipes under pressure. It also specifies the dimensions at the socket and spigot ends of the fitting. The adaptor fittings have one plain socket or spigot for fusion jointing a PE socket or spigot to a PE pipe, or a PP socket or spigot to a PP pipe. The other component of the fitting is metal and is threaded to enable connection to metal pipes, fitting, valves and/or apparatus with pipe threads. The pressuretight joint is achieved by the compression of a gasket. Metal adaptor fittings are intended for use in pipelines for water supply.