There seems to be some confusion about what swelled edges are and are not. So here we go.
The cut edges of cloth fray so the seam is made a certain distance from the cut edges, using a lockstitch, which is the technical name for an ordinary machine stitch. The ¼” to the right of the seam in this photo is known as the seam allowance. The cloth is sewn face to face.
The seam is then turned so that the cut edges are concealed between the layers of cloth. It’s easier to see on this double-faced cloth
The edge is then pressed. This is done on all edges, regardless of whether they will be plain (bluff) or swelled.
Because this pressed edge could puff up if exposed to humidity (steam!) or roll the wrong way, it is traditional to pick stitch the edge to keep these four layers of cloth flat, about 1/16” from the edge (using a backstitch, for those who care). The stitch creates a compression of sorts, keeping everything in place and flat; when done by hand, the goal is usually to be very discrete.
There are machines which replicate this stitch (to varying degrees) known as AMF, Complett and Columbia machines.
If this stitch is done with a lockstitch machine (the same type of stitch used to make the seam itself), it is known as a single stitch. It used to be a feature of all J. Press suits to be single stitched 1/16” from the edge, I don’t know if this is still the case.
On sportier garments, this stitch is sometimes done a certain distance away from the edge, away from the seam allowance. The compression created by this stitch where there are only two layers, next to the four layers created with the seam allowance which is encased by this stitch, creates a sort of bas-relief effect. Think of the effect of quilting a down jacket.
When a single stitch is used instead of a pick stitch, the added tightness of the stitch and additional number of stitches per inch create a more pronounced effect.
The effect is much more evident on heavier goods (conversely, it will be almost nonexistent on lightweight goods).
Done this way, the swelled edge is seen as a somewhat decorative detail to give character to sportier garments. The style in southern Italy is somewhat more exuberant, so it is common to see heavier threads being used to do the stitching there (silk buttonhole twist is common). Having two rows of it is just seen as an additional detail to show that just a little more work has gone into the garment, and most garments that have two rows are done in a very showy, exuberant manner.
