How to Understitch

Ever made a garment where the facing or lining keeps trying to roll out, no matter how many times you tuck it back inside? 

The missing ingredient is understitching, a simple technique that keeps facings and linings exactly where they belong: on the inside of your garment, invisible and well-behaved. It only takes a couple of extra minutes, but it can make a huge difference to the final look of your handmade clothes, and how you feel wearing them. 

This guide will show you exactly how to understitch, when to use it, and how to get professional results. 

What is understitching? 

Understitching is a line of stitching that joins a facing or lining to the garment seam allowances, sewn close to a seam line. Its purpose is to anchor the facing or lining to the inside of the garment. 

It's called "understitching" because it's hidden underneath – you can't see it from the outside of the finished garment. But its effects are very visible: facings that stay hidden, necklines that sit flat, edges that are crisp and neat. 

When to understitch 

As a general rule, if you’re attaching a facing, lining, or bias binding to an edge that you want to stay neatly on the inside of the garment, understitching will probably help. 

You’ll often use understitching on: 

• Necklines finished with facings or bias binding (Samara top, Nell dress + blouse) 
• Armholes on sleeveless tops and dresses (Noa pinafore) 
• Lined garments (Sonny jacket with lining add-on) 
• Lined, faced, or in-seam pockets (Luella skirt, Thea trousers) 
• Waistline facings 

You usually do it after sewing the seam and trimming the seam allowances, but before turning everything right side out. 

How to understitch: Step-by-step 

Step 1: Sew the facing or lining to the garment 

First, attach your facing or lining to the garment using the seam allowance for the pattern. Don’t turn it to the inside just yet. 

Step 2: Trim and grade the seam allowances 

Trimming the seam allowances reduces bulk and helps them sit more neatly on the inside of the garment.  

If your fabric is lightweight, you can trim all layers to about half the width of the seam allowance. Don't trim narrower than 5mm (1/4in) – you need enough seam allowance left to catch in your understitching. 

For thicker fabrics, grade the layers by cutting them to different widths. Staggering the widths prevents a visible ridge from showing through to the right side of the garment. I often grade any interfaced seam allowances narrower than the other layers to make them less bulky. 

Step 3: Clip or notch curved seams 

If your seam is curved (like a neckline), clip or notch the seam allowances so they'll lie flat when turned to the inside. 

Concave curves (curves inward, like a neckline): Clip into the curved area of the seam allowance by making small straight snips at right angles to the seam line, stopping just before the stitching line.  

Convex curves (curves outward, like collar edges): Notch by cutting small triangular wedges out of the seam allowance, being careful not to cross the stitching line. 

Step 4: Press seam allowances toward the facing or lining 

Press the facing or lining and all seam allowances away from the garment. This is important because your understitching needs to catch the seam allowances underneath. 

If your fabric is lightweight, stretchy, or otherwise easy to manipulate, you might be able to get away with finger pressing for this step. Otherwise, use a steamy iron to create a neatly defined seam line. 

Step 5: Stitch close to the seam line 

Now you're ready to actually understitch!  

It’s best to understitch with the right side of the project facing up on your sewing machine, and the seam allowances on the underside. This gives the neatest results because you can pull the seam taut as you sew, which prevents ridges. However, if you’d prefer to see the seam allowances during the process, you may choose to sew with the fabric wrong side up. 

Position your needle 2-3mm (⅛in) away from the seam line, on the facing or lining. You can use the edge of your presser foot as a guide to help keep the stitching line even. If your sewing machine allows it, you can also shift the needle slightly to one side and keep the seam line aligned with the centre of the foot.  

But don’t worry too much if your stitching wanders a little away from this line – it will be on the inside of the garment, so no one is going to see it ;) 

Take your time when understitching, feeling through the layers with your fingers to make sure you’re catching the seam allowances underneath. As you sew, gently pull the fabric taut and away from the seam line – this keeps everything flat and prevents ridges. 

Step 6: Press the facing or lining to the inside 

Once you've understitched, turn the facing or lining to the inside of the garment. The seam line should naturally want to roll slightly to the underside – this is exactly where you want it, so it doesn't show from the outside. Give it a good press, with steam if your fabric can take it. 

What if you can’t understitch the whole seam? 

Sometimes enclosed seams, tight curves, or sharp corners make it tricky to understitch continuously all the way around. 

Don’t worry – just sew as far as you can comfortably reach, back tack, then start again from the other side if needed. Even partial understitching can make a big difference to how neat the finished garment looks. 

Why is my facing still rolling out? 

If your facing still wants to peek out after understitching, here are a few things to check: 

• The understitching may be too far from the seam line  
• The seam allowances may need more clipping  
• The facing may need another good press  
• Lightweight fabrics may benefit from interfacing, stay tape or hemming web on the edge for extra stability 

Video tutorial 

Prefer to learn by watching? We've got you covered:

In this tutorial, we demonstrate the complete understitching process from start to finish, including how to handle curved seams and keep everything aligned as you sew. 

Want to learn more about sewing a polished garment? 

Put understitching into practice along and learn other essential garment sewing techniques in Tilly’s online workshop Learn to Sew an Easy Top.