Are you talking about "double selvage" as seen in Uniqlo, where the inner and outer seams both have selvage edges, or are you talking about in the traditional sense, i.e. Levis redline, where the outer seem has selvage? If the former, I'm not sure myself. The inside selvage may be faked. The latter, j got it right on. On a 29" loom, you can logically cut the sheet in half and you have two pieces with selvage edges that you sew together in the outside leg. It would take a total of four pieces (two full lengths of denim from the loom) to make a pair of jeans. On the 54" loom, it takes one full length of denim. It is more wasteful in the sense that 1) takes much longer to produce the same amount of denim (and moreso because the older looms weave slower), 2) more denim is discarded, so more cotton is consumed.