What are flatlock seams

Short answer: A flatlock is a flat seam where two fabric edges lie side by side instead of on top of each other. The result is a seam you can hardly feel. That is why it is used in clothes that need to sit comfortably against the skin for many hours.

The edges meet on the same level

In an ordinary seam the fabric edges are laid on top of each other and folded, which leaves a small ridge on the inside. In a flatlock the two edges are instead placed next to each other and sewn together with thread that runs back and forth across the join. The seam then lies level with the rest of the fabric. See bamboo sweatshirts for women.

No edge that presses

Because the seam is flat, nothing sticks out to rub against the skin. You notice it most in seams over the shoulders, along the sides or under the arms, where there is a lot of movement. A thick seam can chafe when you move all day. A flatlock stays flat and follows the fabric as it stretches.

Used where skin is closest

Flatlock is most often found in underwear, sportswear and anything worn directly against the skin. Comfort matters most here, as the clothes fit closely and move with the body. A flat seam also makes sense in soft loungewear, whether you are sitting, lying down or moving freely. See bamboo sweat clothes.

What a flatlock looks like

From the outside a flatlock shows as two parallel rows of stitches with thread running back and forth between them. It is easy to spot once you know what to look for. Sometimes it is used as a visible design detail, and sometimes in a colour that blends into the fabric.

Strong and stretchy

Even though the seam is flat, it is strong. The threads cross the join and hold the two edges firmly together. At the same time a flatlock can give when the fabric stretches, so the seam does not snap when you move. That makes it well suited to knitted fabrics that need seams able to move with them.

Flatlock in sweatshirts

In a sweatshirt the fabric is often thicker than in a T-shirt. An ordinary seam in thick fabric can get quite bulky on the inside. A flat seam keeps the join thin, so the top feels softer to wear, even over many hours. See also bamboo sweat for men and women.

Caring for flat seams

Flat seams need no special care, but they benefit from the same gentle treatment as the rest of the garment. Wash at a low temperature, turn it inside out and avoid high heat in the tumble dryer. That way both fabric and seams keep their shape for a long time, and the threads are not worn down needlessly in the wash.

When a flat seam makes a difference

For clothes worn loosely over other layers, the seam matters less. But for clothes that sit directly against the skin for many hours, a flat seam can be the difference between feeling the garment and forgetting you have it on. Check the inside and feel along the seams with your fingers.

Frequently asked questions

What is a flatlock seam

A flat seam where the fabric edges lie side by side, so no ridge forms that could press or rub against the skin as you move.

Where is flatlock used

In underwear, sportswear and anything that sits directly against the skin for many hours at a time, where a thick seam might otherwise chafe and press.

Can you see a flatlock from the outside

Yes, it shows as two parallel rows of stitches. On the other hand you can hardly feel it from the inside, and that is the whole point of the seam.