When you stitch a seam, the stitching is hidden between the layers of fabric. Topstitching, however, is done on top of the fabric and is visible, explain Valori Wells and Carolyn Spencer in "Oh Sew Easy - LIfestyle." It can be purely decorative, or it can help hold layers together. When choosing a thread for the topstitching, you can choose a color that blends perfectly with the fabric or a contrasting color so that the topstitching becomes a decorative touch. You can topstitch by hand or machine. (Lead image from Louella38/Dreamstime.)