This post about the flatlock stitch was originally written by me for One Thimble issue 7

***This post contains affiliate links, which means that if you click on one of the linksI’ll receive a small percentage of your purchase at no additional costs for you, helping me to create more free content for you!

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to and affiliated sites***


Let’s learn how to sew a flatlock stitch!As featured in One Thimble

As you might know, I have a thing for sergers: I need to know everything about them, and I LOVE to share what I learn, because I can’t understand overlockers fear, and I’m here to defeat it!
Overlockers comes with several different stitches: some are perfect for hemming lightweight fabrics, like rolled hem, others can be used to stitch together and finish two (or more) layers of fabric in one time, like the 4-thread overlock, others are great for finishing seam allowances, like the 2 (or 3)-thread overlock.

Today, you’re going to discover all the secrets of the flatlock stitch, made using a regular overlocker (vs. an industrial flat-lock machine).

Flatlock seams’ most noticeable quality is that they have almost no bulk. You can see them in stretch sportswear, where seam allowances in the inside of the garment may hurt athlete’s skin.

Serger Pepper Flatlock Stitch on RTW detail

With this kind of join, there are no seam allowances hanging around, they’re all wrapped inside the seam, with raw edges side by side.

Which side is your favorite?

Flatlock has two nice-looking sides: one with the upper looper’s thread zig-zagging, the opposite with a nice ladder stitch (displaying the needle thread).

Serger Pepper - One Thimble 7 - Flatlock right-wrong-ladder -

If you want the loops showing on the right side of the fabric, sew fabric with wrong sides together; to show the ladder stitch, put it right sides together.

Decorative only?

Flatlock can also be used as a purely decorative stitch, folding the fabric and sewing along that fold. In this case, I would suggest you choose a 2-thread flatlock to reduce the bulk.

Cool Guy Tip: Choose to showcase a nice thread from your stash or, maybe, slip a narrow ribbon under the ladder stitches for a fancy touch!

Straighten the edges first!

Flatlock stitch is sewn without using the blade: the fabric won’t even go next to it so you may technically leave it engaged but I’d suggest you put it out of the way if you’re flatlocking on the fold… better safe than sorry!
If you’re using the flatlock stitch to attach together two pieces of fabric (instead of using it above a fold for ornamental purposes only), you’d better start straightening up the raw edges, making them even, before you start sewing.
Serger Pepper - One Thimble 7 - Flatlock 1 -

Pro Tip: Use a rotary cutter and mat*.

Seam Allowances

You may need to trim away seam allowances, depending on how wide are the ones already included in your pattern pieces: you’re going to stitch at above 1/8-3/16″ from the raw edge.

Choose the right foot

While you can use a standard serger foot (on the right in the picture below) to sew a flatlock seam, it will be way easier if you choose an adjustable blind hem foot* (pictured on the left). Make sure you choose a replacement foot that is compatible with your serger brand and model!

Serger Pepper - One Thimble 7 - Flatlock serger feet -

  1. If using a standard foot, align the fabric raw (and trimmed) edge between the needles (test your serger to identify some reference points, like the little hole you see in mine).
  2. If using an adjustable blind hem foot, try on scraps to decide how much you need to move the adjustable slider to the right, then close the tiny screw on it: no more guessing!

Serger Pepper - One Thimble 7 - Flatlock blind hem foot guide -

Generally speaking

A flatlock is sewn leaving loose the needle’s thread tension (around zero) because you need to pull the fabric to open the seam after you’ve sewn it. This way it will lay flat and the needle’s thread won’t pull the fabric.

Tip: this is a stitch that needs a little fiddling to be perfect: you’d better tune it on scraps before you actually sew on our finished garment. If you spend that little time practicing you’ll be amazed from results, substantially improving your sewing level.

Flatlock seams are rad and I want to add them everywhere!|tweet this|

When sewing a flatlock, opposite to a regular overlock, you need to serge leaving the stitches hanging off of the fabric; this will leave enough room to the fabric to lay flat, once you’ve gently pulled it on the sides.

Serger Pepper - One Thimble 7 - Flatlock on the edge -

Pro Tip: Add strength (but also a little bulk) to a flatlock by sewing a regular seam first, then fold it along the seam, enclosing seam allowances. Flatlock on top of it. Press flat, then trim seam allowances close to the flatlock stitching.

2 vs. 3

Let’s compare a 3-thread flatlock to a 2-thread flatlock, and have a glimpse at how to create them.
Since each overlocker is different and changing fabric/thread combo will make settings change (sometimes significantly), consider the following settings as starting points.

Note: Next to each item there’s a setting range, in brackets the settings I used for flatlock stitch you see.

3-thread flatlock

  • It’s the stronger between the two;
  • it uses more thread.

Serger Pepper - One Thimble 7 - Flatlock right-wrong-ladder -

left needle 0-3 (0.5) pink
upper looper 2-5 (4.5) violet
lower looper 6-9 (7) purple

differential feed 1-1.5 (1)
stitch length = your choice
left or right needle (usually the left one, but you can experiment)

2-thread flatlock

  • It’s the weaker between the two, not supposed to be used for piecing but for decoration, sewn on the fold;
  • less thread consuming.

Serger Pepper - One Thimble 7 - Flatlock 2-thread -

To convert your overlocker to a 2-thread stitch, you need to snap-on a converter to your upper looper, to by-pass it.

Each brand/model has a different one but they all serve the same purpose – check your manual if you’re in doubt.

Serger Pepper - One Thimble 7 - Flatlock 2 thread converter -

left needle 0-3 (0) pink
lower looper 6-9 (6) purple

differential feed 1-1.5
stitch length = your choice
left or right needle (usually the left one, but you can experiment)

Keep in mind that:

  1. the loop side will be the looper’s one
  2. the ladder is made with the needle thread.

Choose threads wisely, depending on which side you’re going to show to the world!

Cheat flatlock stitch, anyone?

Flatlock seams are cool but they need some testing time to become perfect. What if you want to achieve a similar look, spending a fraction of the time?

  • Start with a regular 3-thread overlock (with balanced tension settings) sewn putting wrong sides together, then press the seam to one side.

Serger Pepper - One Thimble 7 - Faux- Flatlock -

  • Edgestitch it to the fabric below it using a thread color matching with the fabric.
  • Going straight will be easier using an adjustable edgestitch foot on your sewing machine, but it can be done with a standard foot and a good eyeballing.

Serger Pepper - One Thimble 7 - Faux- Flatlock edgestitching -

  • Let’s compare it to a flatlock seam (on the right):

Serger Pepper - One Thimble 7 -Flatlock vs. Faux- Flatlock -

Hemming with flatlock

An interesting use of the flatlock is for hemming. It works like a charm for an almost invisible hem, showing ladder stitch on the right side of the garment (either in a matching color to almost disappear or in a contrasting color with decorative purpose).
The best part is that you’re going to hem and finish in a professional looking way the inside too, all at once (and who doesn’t like time-saving tricks?).

Here too, using the adjustable blind hem foot will be a huge life-saver, but you can absolutely do without it, using a standard serger foot.

  1. Fold the fabric to the wrong side for the hem width, and press the fold.
  2. Fold back the fabric, aligning the new fold right above the raw edge.
  3. Serge on this edge, using the flatlock stitch (either 2 or 3 threads, I used the 3). Try to catch with the left needle as little fabric as you can, for even more invisible results.

Serger Pepper - One Thimble 7 - hems -

  1. Unfold the fabric, gently pulling it to set stitches, press and admire your lovely hem inside (on top, in the picture) and outside (on the bottom, in the picture).

Using a matching thread the result will look more precise, blending on your fabric. Here, I have used contrasting thread to help you see it and I would have pulled just a tiny bit more, to better set the needle’s thread:

Serger Pepper - One Thimble 7 - flatlock hem inside and outside -

Now you know (almost) everything about the flatlock stitch. Please ask questions, if you have any: I’ll do my best to help you!

Where would you use the flatlock? Do you have a project in mind to test it? I have “just a few” 😉