<b>Pipes</b> Pipes and pipelines are classified according to their nominal widths (inner diameter or inside diameter of the pipe) and their permissible operating pressures. The nominal width is specified in accordance with EN ISO 6708 using the designation DN (French: “diamètre nominal”) followed by a dimensionless number that roughly corresponds to the inner diameter in millimeters. It is a parameter for parts that belong together, such as pipes, flanges, valves and fittings.