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Understitching is to keep a seamed edge from showing and keeps a facing or lining from flipping out. I know this might not make much sense right now, but hopefully it will after you see the pictures. When I first learned about understitching, I was completely lost until I saw it done.
Anyway, start with a #5 foot or the foot with the little fence.
How To Understitch
Position your fabric under the foot with the fence right on the seam. Move your needle to the facing or lining piece. In my example, the blue checked fabric is what will be seen from the outside. The green checked fabric is the facing, lining, or inside fabric. The inside fabric will not be seen when being worn.

How To Understitch

Make sure that all the seam allowance underneath is pushed to the side that is the inside fabric.
See seam allowance is on the right, and my needle is on the right.
How To Understitch
Sew along the entire seam. It should look like this.
How To Understitch

The inside should look like this. The dark thread is the original seam, the pink thread is the underside of the understitching. All the seam allowance is pushed to one side and sewn down.

How To Understitch

When the seam is folded to be finished, none of the inside fabric shows. I am folding along the seam, but the understitching keeps the inside fabric from showing.

How To Understitch
This is the view from the inside. The seam and stitching line shows.
How To Understitch
This is my attempt to show you both sides at once. The crease of the fabric is right next to the seam.
How To Understitch
Here is an example of the neckline of a dress. The blue fabric is the lining and the patterned fabric is the fashion fabric. The lining is held down by the understitching. The peach fabric is a little ruffle.
How To Understitch

Understitching can also be used when there is something between the fashion fabric and lining like ruffles or piping. This is the neckline of my photoshoot dress. The understitching keeps the piping lie flat and the lining inside.

How To Understitch
I hope that this makes sense! Good luck with your understitching!

Author: Heather