Preferred metric sizes

Given that even ISO 262 specifies a fairly large set of diameters, a much simplified set of preferred diameters was developed by one of the lead designers of ASME Z17.1 and ANSI B4.2, Knut O. Kverneland, to reduce the list to 6 preferred sizes, and another 6 intermediate supplementary sizes.

Each nominal diameter specifies different ID minimum and maximum values.

Nominal diameter, abbreviated DN (diamètre nominal/Durchmesser nach Norm), is the designation system specified by ISO 6708 for specifying the diameter of trade sizes of metric pipework components, and is the metric equivalent to Nominal Pipe Size.

The complete set of DN values allowed by the standard are: The number following the DN is a nominal value that is roughly the number of millimeters of a circular feature on the connection point of the pipe, fitting, coupling, etc., but often differing by a noticeable amount.

Note that the actual internal diameter varies depending on the pipe wall thickness.

Use of preferred metric sizes is common in engineering designs
Champagne bottles in various sizes, probably 200 ml, 375 ml, 750 ml, 1.5 L, 3 L, 6 L, 9 L, 12 L and 18 L.
Shelf where the shelf bearing holes are placed with 32 mm distances from center to center, giving flexible choices for shelve positioning.
Cross section of a wooden joist layer , where 6 M (or 6 modules) indicates a distance of 600 mm.
A-series paper sizes .
Machine screws with different thread diameters
Hex key set with metric sizes from 2 mm to 10 mm according to sizes for use with ISO 262 screw sizes (except 3.5 mm for M6 internal set screw)
Garden hose with mouthpiece
Butterfly valve in a large DN-size .