What is an overlock seam

Short answer: An overlock seam joins two layers of fabric and wraps the edge in the same step. It is the most common seam in knitted fabrics, because it holds the pieces together, closes the edge and stretches with the fabric.

Two jobs at once

An overlock machine sews the layers together and finishes the edge at the same time. While the needles stitch, a small blade trims the raw edge, and the threads wrap around it. The result is a seam where the fabric cannot fray, even after many washes. That is why you find it inside most knitted tops and underwear.

Why knits need it

Knitted fabric is built from loops that give when you move. An ordinary straight stitch from a sewing machine lacks that stretch, and when the fabric is pulled, the thread can snap. The overlock is made for exactly this kind of fabric. It gives when you pull a top over your head or bend down, then settles back.

The seam can stretch

The overlock uses several threads that run in loops across the edge. The loops open out when the fabric stretches and fall back into place when it relaxes. This is why the seam follows the fabric instead of fighting it. See bamboo slips, where the seam has to move with the body all day.

It forms a small ridge

Because the layers sit on top of each other with thread wrapped around them, a small ridge forms on the inside. In thick fabric you barely notice it. In thin fabric you may feel it against the skin or see a faint line when the fabric sits close. See bamboo hoodie for women, where fuller fabric hides the ridge.

Where to find the overlock

Turn a knitted garment inside out and look at the side seams and sleeves. You will usually see the overlock as a dense row of loops along the edge. It typically joins front and back, sets in sleeves and closes the shoulders. Hems at the bottom and cuffs are often sewn with a different seam that holds a fold in place.

Overlock and flatlock

Overlock and flatlock are often confused, but they lie differently. The overlock stacks the layers and forms a small ridge. The flatlock places the layers side by side, so the seam lies completely flat. Flatlock is often used in clothes worn close to the body for long periods. Overlock is the most widespread because it is simple and holds up well.

When the ridge bothers you

If you feel the seam against your skin, it usually comes down to how close the garment fits. A size that is too tight presses the ridge into the body. Try a size where the fabric rests lightly on the skin without pulling. For underwear meant to fit close, look for styles with few seams. See bamboo bra.

Making the seam last

An overlock is strong, but it can wear if clothes rub hard in the washing machine. Turn garments inside out, wash at a low temperature and do not fill the drum completely. Avoid pulling at the seams while the clothes are wet, since fabric and thread give more then. Dry them flat so the seams do not twist.

Frequently asked questions

What does an overlock seam do

It sews two layers of fabric together and finishes the edge in the same movement, so the fabric does not fray in the wash. It is the most common seam inside knitted clothing.

Why is overlock used on knitwear

Because the seam can stretch with the fabric. An ordinary straight stitch would snap when the fabric gives, while the loops of the overlock move with it and settle back.

What is the difference between overlock and flatlock

Overlock stacks the layers and forms a small ridge. Flatlock places them side by side and lies flat, so you feel it less against the skin.