One of the most powerful tools in your sewing arsenal is underlining. Couture garments almost always incorporate an inner layer of fabric (sometimes more than one layer) that supports the fashion fabric. Once joined, the layers work as one.
Underlining’s role in a garment can be pronounced or subtle. It provides arm base for details that are attached to the fashion fabric (lace appliqués, trims, fasteners, and so on). It adds polish by camouflaging construction details (boning channels, seam allowances, darts, and hand-applied stitches, for example), while making others stand out (filled pleats and gathers). It reduces wrinkling and absorbs perspiration, and it can even be used to shift the color of the fashion fabric.
Here, Susan Khalje explains how to select the correct underlining, and how to mark it and join it to the fashion fabric.
From Threads 154