90LR ELBOW

It's got something to do with the 30-60-90deg set square that draughtsmen use on their drawing boards, Chief. A piping isometric drawing needs these angles, and it is easier to get pipes past each other when the elbows are 90deg rather than any other angle. A 72deg bend wouldn't get much use, for example.

Less friction/pressure drop versus a short radius elbow (less erosion in slurries, too)

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