The iron pipe size standard came into being early in the 19th century and remained in effect until after World War II.
[1] The inside diameters under IPS were roughly the same as the more modern Ductile Iron Pipe Standard (DIPS) and Nominal Pipe Size (NPS) Standards, and some of the wall thicknesses were also retained with a different designator.
In 1948, the DIPS came into effect, when greater control of a pipe's wall thickness was possible.
The IPS number (reference to an OD) is the same as the NPS number, but the schedules were limited to Standard Wall (STD), Extra Strong, (XS) and Double Extra Strong (XXS).
Different definitions exist for XXS, but it is generally thicker than schedule 160.