We don't use the word 'fittings' for this. The elbow is the whole thing. If we look at Andy's picture in #11, the elbow consists of four sections with three joins, which are mitered joins. Mitred means they have these corners in them. This makes it possible to build the elbow using simple flat sheets of metal, rolled into cylinders and then cut at an angle.But in the table, why do they call it elbow, not fittings?
Smooth means there are no corners in the elbow. As in the lower picture in #9. You can't make a smooth curve from joining flat sheets of metal together.So "smooth" means no fitting in the elbow?
Both types of elbow are bought as a single piece.So smooth means one curve/turning piece?