Rectangular Duct
A straight length of rectangular duct: one blank wrapped round to one seam, two L-shaped pieces, a U and a flat side, or four sides. A straight length of rectangular duct folded from sheet: in one piece wrapped round to one seam, in two L-shaped pieces, as a U and a flat side, or as four flat sides.
How the rectangular duct is developed, with a worked example · Folded work