Detached Buttonhole Stitch
The foundation is two long stitches the length of the area to be covered. Work Buttonhole Stitch, as close together as possible, over the two threads, but not through the fabric. The next row is worked through the loop of the Buttonhole Stitches above, but not over the two foundation threads or through the fabric.
Mary Thomas's Dictionary of Embroidery Stitches, p 22-23