The first standardization of cast iron water pipes in Britain occurred in 1917 with the publishing of BS 78.  This standard specified a dimensionless nominal size, which approximately corresponded with the internal diameter in inches of the pipe, and four pressure classes, Class A, Class B, Class C and Class D, each with a specified wall thickness and outer diameter.  It is noted that the outer diameter is identical between classes with the exception of sizes 12 to 27, where Classes A and B share one diameter and Classes C and D have another, larger diameter.
BS 78 was finally superseded when the UK harmonised with incompatible European standards, however, the specified outer dimensions continue to remain in effect (albeit in metric form) as the standard pipe outer diameter for ductile iron pipe in Australia and New Zealand through the descendant specification, AS/NZS 2280.