The Standard for Pipe Sizing came into being early in the 19th century when lighting fixtures were supplied with natural gas as energy. The pipe was supplied from the brass mills in long tubes. The tubes were specified buy their Internal Diameter (IPS is believed to originally be an acronym for Internal Pipe Size). The Internal Diameter Opening has a direct bearing on the amount of gas that could flow through the pipe. Most fittings were brazed together to prevent gas leaks so threading was not common. Early in the 20th century when Electric Lighting became popular electricians would take gas fixtures and convert them buy removing the gas nozzle and replacing it with a light socket. After time the electrical industry made their own tubes thinning out the wall thickness on the inside for more room for wires to pass through and retaining the outside diameter since that size tube was already a standard. For Convenience threads were added to replace the need for brazing. Eventually Iron was used instead of brass to reduce costs and that's when the Iron Pipe Size Standard came to be.