I read in an old sewing book about a “channel seam”. You baste your two pieces of fabric together with a fairly wide seam (about 2 cm) then press it open. On the reverse side, you pin and baste a strip of contrasting fabric (right side down). Then, on the right side, you stitch two narrow seams, equidistant from the first seam. You then unpick the original basting. What you are left with is two flaps covering a contrast material. It sounded interesting so I tried it as what will be my final decorated seam in my sampler.
The channel seam is at the top. I used two pieces of plain black fabric with one of my machine stitched strips for the contrast underneath. I did a few hand stitches to hold the flaps alternately together and apart at intervals along the seam. See detailed photograph.
ceesew 