What is the purpose of underlining? What should I use and how do I apply it?
Underlining, a layer of fabric applied to an entire garment piece, can serve many purposes. It's often used on white or light colored fabrics to prevent seam show-through. It can add firmness and weight to a lightweight fabric and opaqueness to a sheer fabric or change its color. It can reduce wrinkling, making it an excellent choice for linens. It may absorb perspiration and protect the fashion fabric so it's often used in evening and bridal gowns.
Interfacings that can't be applied directly to a fashion fabric can often be applied to an underlining. Silk organza and cotton batiste are excellent choices for soft support; muslin or flannel give a firmer support. Experiment to decide on the best choice for the fashion fabric and purpose.
Cut underlinings from the same pattern pieces as the fashion fabric. Mark the underlining and position it on the fabric wrong side. Align the grainlines and pin the layers together at the seam allowances. Turn the piece over and be sure the fashion fabric is smooth.
Adjust, if necessary, then baste the layers together from the underlining side. (Hand stitching gives greater accuracy.) Use a single silk thread strand; it pulls out more easily than other thread types. Sew on or just inside the seamline, stitching each edge separately and going all the way to each corner, crossing threads at the corners (Figure 1).
Baste each edge separately, crossing threads at corners.