This International Standard gives the definition of DN (nominal size) when applied to components of a pipework system, as specified in those standards which use the DN designation system. NOTE 1 Other methods of size designation are also used, e.g. products having threaded, compression, solder or welded end connections or products designated by NPS (nominal pipe size), OD (outside diameter), ID (inside diameter), but these are not defined in this International Standard. This International Standard gives a list of preferred DN values.
GSO ISO 6708:2008 history
2008GSO ISO 6708:2008 Pipe work components – Definition and selection of DN ( Nominal Size )