​Why All Garment Facings Should Be Understitched

One of the most important elements of sewing a good quality facing application is the necessity to understitch the facing layer. Understitching is a stabilizing straight stitch applied next to the seamline through the facing layer and seam allowance layers underneath. 

The understitch is applied after the facing has been stitched to the edge it finishes. Once the seam allowance is notched and trimmed as needed, the seam is ironed with the facing and all seam allowance layers directed away from the garment. Ironing the facing in this position stabilizes it for machine stitching. 

The facing and all seam allowance layers underneath are then topstitched next to the seam, at about 1/8" from the seamline. This is called understitching and it serves a purely functional purpose - it should not be visible on the garment's right side once the facing is completed.

Understitching serves two important purposes: It smooths and flattens the neckline edges, regardless of its shape (whether curved or straight), and it prevents the facing compartment from rolling to the outside of the finished garment by keeping it flush against the garment's wrong side. 

Understitching should be applied along all garment edges that are clean-finished with a facing, including faced front openings, hemlines, armholes and neckline edges. For example, an all-in-one neck/armhole facing (pictured above) is used to finish the garment's neckline and armhole edges simultaneously. In this case, understitching should be applied along both armhole edges and the neckline.