If you have a habit of ignoring the instructions for understitching -I know I used to have- I'm here to tell you that you're never again allowed to skip this step! You'll thank me for it since it makes a massive difference to the quality of your finish.
The main reason for understitching is that it stops the facing or lining from rolling to the outside of the garment. It also stabilizes the edge and gives it a nice, sharp finish. It's used on all neckline and armhole facings, linings, pockets, whatever you can think of that is in the inside of the garment and not supposed to show on the outside.
On a curved edge you'll need to trim the seam allowance before understitching. This will allow the seam allowance to lie flat. You can clip into the seam allowance at regular intervals.
Alternatively, just trim the entire seam allowance to about 1/4" ( 5 mm ) or even less along sharp curves.
Press the seam open, pressing the seam allowance towards the facing. Stitch very close to the seam line -about 1/8"- attaching seam allowance to facing.
I do this from the wrong side simply because I have an obsession with trimming my seam allowances ridiculously narrow. If you're unsure of how neat a finish you can achieve from the wrong side you should stick to stitching from the right side. Make sure to catch the seam allowance underneath.
On the wrong side the seam allowance will be firmly sewn onto the facing.
Right side will have a neat stitch along the edge of the facing.
Turn facing to the inside so that the shell fabric rolls slightly onto the wrong side. You can now start to appreciate your efforts: You should notice that the facing will lie like this naturally because of the understitching! Press carefully in place.
Match seams on the inside and do a few stitches along the seam from the right side to help the facing stay in place. (See "stitching in the ditch" -tutorial)
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