The 4-thread overlock is the workhorse seam of knitwear production. It does two things simultaneously: it joins two panels of fabric together and finishes the raw edges to prevent fraying — all in a single pass through the machine. For knitted fabrics like jersey, it's the default choice for side seams, shoulder seams, and sleeve attachment.
The "4-thread" refers to how many threads are used to form the stitch. Two needles create the joining stitch while two loopers wrap the edges. The result is a seam that is strong, clean, and — critically for knits — elastic enough to stretch with the fabric without snapping.
When your tech pack says "main seam type," the 4-thread overlock (ISO 514) is the correct answer for any jersey or knit garment. It's not a stylistic choice — it's the production standard.
Why knits need elastic seams
Woven fabrics don't stretch much. A standard lockstitch seam (the kind you'd use on denim or a woven shirt) works fine for them. But jersey stretches — sometimes 50–100% of its width. If you sew jersey with a rigid lockstitch, the seam breaks the moment the garment is pulled on or stretched during wear.
The overlock stitch is constructed as a loop, not a straight line. That loop structure gives the seam built-in elasticity — it can stretch with the fabric and return to shape without breaking. This is the fundamental reason overlocking is used for knitwear, not just because it looks clean.
4-thread vs other overlock types
Production specs for your tech pack
| Spec | Value | Notes |
|---|---|---|
| Seam type | 4-Thread Overlock | ISO 514 classification |
| Stitch density | 10–12 SPI | Stitches per inch — standard for jersey |
| Thread | Polyester Tex 27 | High tenacity, resistant to abrasion |
| Seam allowance | 1 cm | Standard for jersey garments |
| Application | Side seams, shoulder seams, sleeve set | Wherever two panels are joined |
| Needle | Ball Point SES 70/10–80/12 | Required for knit fabrics |
In FlatLabs PRO, selecting the 4-thread overlock automatically populates your construction notes with the ISO 514 reference, stitch density, thread spec, and needle type — ready for your factory.
Where it appears in a t-shirt
On a standard t-shirt, the 4-thread overlock is used for the shoulder seams (joining front and back panels at the top), the side seams (closing the garment from underarm to hem), and the sleeve set (attaching the sleeve to the armhole). These are the structural seams — the ones that take the most stress during wear.
The hem and neckband are typically finished differently — the hem with a coverseam, the neckband sewn with a coverseam after the rib is attached. The overlock is specifically for where two panels meet edge to edge.