How to Sew an Understitch on a Neckline or Armhole

This sewing tutorial will teach you how to sew an understitch on a neckline or armhole, or even an inseam pocket.

When is understitching used, and why?

Understitching is used to sew the seam allowance of a garment to the lining or facing (which can also be a bias facing, aka bias binding). It’s usually used on the neckline, armhole, or inseam pocket. Of course, once you’re familiar with the concept, you will probably find other applications for it as well.

The understitching is invisible from the right side and neatly keeps the lining or facing from showing. It also adds some structure to the section.

In this tutorial, I will show you how to sew an understitch on the neckline of a lined bodice, as well as on an armhole with bias binding.

The process is basically the same for any of these applications. So first I’ll explain the steps in detail for the lined bodice. Then I’ll show steps and pictures of the armhole with bias binding. Last, I’ll describe how to sew an understitch on an inseam pocket.

Supplies and materials

• Sewing machine
• Fabric, woven or knit
• Matching thread (I really like Gutermann Mara for an all purpose thread)
• Scissors
• Pins
• Iron and ironing board

How to sew an understitch on a neckline with a lined bodice

Sew your seam

First, sew the seam that you are going to understitch normally. In this case, I’m showing how to understitch the neckline of a lined bodice that will have buttons on the back. In order to get the understitching really nice and close to the end of the seam, I like to sew only the neckline at this point. We will sew the center back (where the buttons will be) after the understitching is complete.

Trim your seam allowances and notch

Trim the seam allowances down to 1/4 inch. You will need to notch (or clip, technically) the seam allowance all along the curve. These are just little snips that come close to the stitching line, so that the seam allowance can pull open when it’s turned and allow the garment to sit flat. The tighter the curve, the more notches you will need. Usually they will be 1/2 – 1 inch apart or so.

Iron or press seam allowance towards lining

This step is optional. You will be manipulating the seam allowance towards the lining as you sew, so ironing now might make it a little easier.

Edgestitch along the lining

Now lay your garment right side up, and make sure the seam allowance is pressed towards the lining (laying underneath the lining).

You will be sewing on the lining, very close to the edge, about 1/16 inch.

You are NOT going to start your stitching right at the beginning. Instead, start and stop stitching a little more than your seam allowance from the end. So, for a 1/2 inch seam allowance, I start about 3/4 inch from the end. If you start and stop right at the end, you won’t be able to sew your center back seam properly.

Of course, if you’re sewing understitching on a neckline that doesn’t have an opening, you won’t need to worry about this. In that case you can just understitch all around the circle until you get back to where you started.

This next picture is getting a bit ahead, but it shows what the understitching will look like from the inside, and how the stitching doesn’t start or stop on the ends.

As you sew, you need to make sure sure that you are pulling the main fabric and the lining away from each other, and keeping the seam allowance underneath the lining.

The lining will want to fold over onto the main fabric, especially where the curves are tighter. And the seam allowance will want to slip over to the other side every once in a while. So sew a couple inches and keep checking and adjusting as needed. You have to really keep a close eye on it, because it can be easy to miss and if you sew over a fold, you’ll need to seam rip. All that said, once you have this down, understitching only takes a couple extra minutes. So don’t be put off by the warnings!

You can see below how the lining on the right side wants to fold over to the left side.

Sew the center back seams

Now you can sew the center back seams. Because you didn’t get too close to these edges, you’ll be able to sew them together properly. Then turn it to the right side and give it a good iron. You’ll see how the understitching basically forces the main fabric to sit a little higher than the lining, effectively hiding the lining.

Trim the excess lining fabric (optional)

You will notice that there will be a little bit of excess lining fabric along the armholes and bottom edges because of how it’s pushed lower than the main fabric. For best results, you can trim this away.

Then finish the rest of the bodice according to your pattern’s instructions.

How to sew an understitch on an armhole with bias binding

This process is essentially the same as sewing understitching on lining, but it looks a little different, so I thought I would show some pictures. You can also do this on a neck with bias binding.

Sew your seam

When sewing bias binding to an armhole, you can either do it with the side seam already sewn, so that you’re basically working inside a circle. Or you can do it before, which makes the process a lot less awkward. Both ways have their pros and cons. For this tutorial, I’m sewing it to an open armhole before I sew the side seam.

So, sew your binding to the armhole, right sides together.

By the way, I have a tutorial on how to sew shirring (the stretchy gathering stitches on this dress), if you’d like to check it out!

Trim your seam allowances and notch

Understitch along the lining or facing

Because we need to sew the side seam after we do the understitching, we are again not going to start or stop stitching on the ends, but about 3/4 inch in.

Just like with the lining, make sure that you are pressing the seam allowance towards the bias binding and that nothing is folding over to the wrong side.

Sew the side seams

Although the understitching is done, I thought I would explain how to sew the side seams as well, because it can be a bit tricky. This is because you really want to get your binding nicely lined up for this method to look good.

Pin your side seams, placing pins exactly where the seams that connect the dress to the binding are. Check from the right side that they are perfectly lined up. Then sew your side seams, carefully sewing over your pins where the seams are to make sure they stay perfect.

Finish your garment according to your pattern’s instructions

You can then fold your bias binding to the inside twice and edgestitch. Then finish sewing your garment according to your pattern’s instructions.

How to sew an understitch on an inseam pocket

For an inseam pocket, you would simply sew one pocket piece to one pant leg piece, right sides together. Then turn to the right side and edgestitch along the pocket, making sure the seam allowance is pressed towards the pocket. In this case, you can start and stop right at the ends. Repeat on all pocket and leg pieces, then sew together according to your pattern’s instructions.

And that’s how you sew an understitch!

I hope you found this tutorial helpful, and please let me know if you have any questions about this process. Thanks for following along!