Understitching is stitching the lining or facing to its seam allowance. It is done on pocket openings, necklines, and armholes to force the facing or lining to roll cleanly to the inside and stay there. It is not visible from the outside of a garment.
It can get confusing the first time you understitch because it can be difficult to orient all of the terms and instructions with the actual pieces of fabric in your hands. Here are some photos that I hope will help!
Start by sewing your seam like normal. This means place the facing/lining right sides together with your main fabric. Open up the seam and press open, making sure to press the seam allowance toward the facing/lining and away from the main fabric.
You can understitch from the top or bottom–it’s up to you. All you need to do is stitch the seam allowance to the facing/lining around 1/8″ away from the original seam line. Whichever side you sew from, make sure you’ve caught the seam allowance in the understitching line.
After understitching, turn the facing/lining to the inside of the garment and press. See how the facing/lining easily rolls to the inside and isn’t visible from the outside now? It’s a clean way to finish these types of edges and makes everything stay in place without topstitching.
Here’s how understitching would look on a pant pocket. The facing and lining will now roll cleanly and easily to the inside. Cool trick, huh?!