ASME
ASME B16.9
Factory-made Wrought Buttwelding Fittings
Dimensions and tolerances for elbows, tees, reducers and caps. It fixes centre-to-end dimensions so a fitting can be ordered from any maker and still fit the spool the drawing describes.
This is an explanation, not the standard. AlpineTutor is not affiliated with ASME and reproduces no part of the document. Always work to the current edition of the standard itself and to your project specification.