Fittings!!!! - no more simple than changing a fluorescent light bulb for the inexperienced!
Yes, it does refer to sixteenths of an inch. Who decided this I do not know.
Now, for the grand prize, someone tell us of the origins of pipe sizes!
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Who is so ignorant as not to know that knights-errant are beyond all jurisdiction, their only law their swords, while their charter is their mettle and their will is their decrees? Don Quixote