What is a coverlock seam

Short answer: A coverlock seam is the double row of stitching you see at the bottom of a T-shirt. It holds a folded hem in place and stretches with the fabric, so the hem does not snap when the garment is pulled on.

Two rows of stitches on the front

The coverlock sews two parallel rows of stitches on the front and locks them together with a thread on the back. From the outside you see two neat, straight lines. On the inside there is a web of thread running back and forth between the rows. That web holds the hem and lets it stretch. See bamboo sweatpants for men.

Used for every folded edge

Sleeve hems, bottom hems and waistbands are usually sewn with a coverlock. The edge is folded over, and the seam holds the fold so it does not roll up. This gives a clean finish without loose threads that lies flat against the body. You find it almost everywhere a knitted garment ends. See bamboo joggers for men.

It gives with the fabric

Because the stitches run in loops, the seam can stretch without snapping. Pull on a sleeve hem and the loops give way, let go and they settle back. An ordinary straight stitch could not do this, and the thread would break the first time the hem was stretched. This matters most at openings that go over a hand or foot. See socks for men.

Coverlock and overlock

The overlock joins two layers of fabric and closes the edge on the inside. The coverlock does not join two pieces. It holds a fold in place and shows on the front. Many knitted garments use both: overlock at the sides and coverlock at the hems. To tell them apart, look at the front. Two straight rows of stitches mean coverlock.

Why the hem lies flat

A good coverlock keeps the fold flat, so the hem neither ripples nor rolls. That requires the thread tension to suit the fabric. If the thread is too tight, the hem draws in and ripples. If it is too loose, the fold can slip. A hem that still lies neatly after a few washes was sewn with the right tension.

When a hem starts to roll

If a bottom hem rolls up after some washes, the fabric has often shrunk slightly or been stretched. Do not iron or press the hem hard to fix it. Instead, ease it gently back into shape while the garment is damp and let it dry flat. The fold will usually lie neatly again without putting extra strain on the seam.

Caring for the seam

The coverlock sits where clothes wear the most: at sleeves, trouser legs and hems. Turn garments inside out before washing, so the seam does not rub directly against the drum. Wash at a low temperature and avoid wringing wet clothes hard. Dry them flat so the fold does not twist out of shape.

What to look for

When you look at a new garment, turn the hem over and check the seam. The stitches should sit evenly, with no loose threads or gaps in the row. Pull lightly on the hem and see whether it gives and settles back. A coverlock that stretches without puckering is sewn to last in everyday use.

Frequently asked questions

What is a coverlock seam

A seam with two parallel rows of stitches on the front that holds a folded hem in place and can stretch with the fabric without snapping.

Where do you see it

At the bottom of T-shirts, on sleeve hems and on waistbands. It is the most common hem seam in knitted clothing and shows as two straight lines on the front.

Why does the seam not snap

Because the thread runs in loops rather than straight lines. The loops open out when the fabric stretches and settle back into place afterwards.