Understitching

In addition to giving your garment a finished, professional look, understitching actually makes the construction process easier and more efficient. As I tell students, it's the one, most important, most effective thing you can do if you want to take your sewing to the "next level."

Understitching causes the fashion fabric to roll slightly to the inside, keeping linings, facings, and undercollars from peeking out to the front (a screaming indication of a "home made" garment.)
Here, I've used a cream lining in a black corduroy bodice. The understitching is the cream stitching just inside the neckline edge. It does not show on the outside of the garment.
Understitching also makes pressing easier. It positions the seam just right before you press and you're less likely to end up with burnt fingers.

How to understitch

First trim interior seam allowances to 3/8" or less. Grade the seam allowance layers so the longest layer will abut the public side of the garment. Clip if necessary, clipping on the bias and staggering clips between layers.

Place work right side up under the pressor foot so that lining (red), or facing or collar is to the right of the needle and fashion shell (black) only is to the left. Here is the key...all seam allowances underneath the work must be underneath the lining side. You will topstitch a scant 1/8" from the ditch of the seam, through the lining and all the seam allowances underneath.Below is a better view. In this case, the facing is a narrow bias strip. Note the trimmed seam allowances under the facing side.From the side, the understitching process looks like this:
Back to the red and black....Gently separate the lining and the face fabric as you sew. And check often to make sure that all those seam allowances are to the right, under the lining, under the stitching. Sometimes when you are understitching a curve those allowances will tend to pop back over to the left of the seam.

Here is the understitched seam (left) - and how it looks before pressing (right). Note the red lining is already rolling to the inside and a little ridge of the black forms at the edge. It will be very easy to press. No struggling to open the seam.

Sometimes you can only understitch to a certain point, for example towards the zipper of a faced waist. Just go as far as you can and take a couple back stitches or pull the threads to the wrong side and tie them off (yeah, right.)

When you must understitch around a corner, veer into the ditch of the seam just at the point of the corner, then back out again and on your way.


Where to understitch?

Understitch linings, facings, and undercollars.

Understitch lapels on the jacket side above the roll line and the facing side below the roll line.

Understitch faced circular ruffles or scallops.