Understitch or Understitching

A line of straight stitching sewn just beyond the seam line of two pieces that have been sewn together to create a seam along an edge. One of the pieces is the outside of your project, the other is the inside. In order to keep that seamed edge sharp and clean, you understitch the inside piece to the seam allowance so it won’t roll to the outside along the edge seam and look all messy and unprofessional.