Definition
Understitching is a line of stitching that holds the seam allowances to a facing or lining, close to the seam, so the facing rolls inside and stays invisible.
After you sew a facing or lining to an edge, the seam tends to roll toward the outside, showing the facing from the right side. Understitching fixes that permanently and is one of the small steps that separates a homemade-looking neckline from a professional one.
How to do it:
- Sew the seam, then grade the seam allowances and clip the curves.
- Press both seam allowances toward the facing.
- From the right side, stitch on the facing, 2 to 3 mm (about 1/8 in) from the seam, catching both seam allowances underneath. The garment itself is not stitched.
- Turn the facing to the inside and press. The seam now rolls slightly toward the inside.
On tight curves, spread the seam with your fingers as you sew so the allowances stay flat beneath the foot. Around a full neckline or armhole you may not reach every point, for example at narrow shoulder ends: understitch as far as you can and press the rest firmly.
Understitching works on facings, linings, waistband facings, pocket openings and bias facings. It never shows from the outside, unlike topstitching, which is why it can be used even on the most refined garments.