The coverseam is what makes a t-shirt look finished. Flip the hem of any quality t-shirt and you'll see two parallel stitch lines on the outside and a looped cover stitch on the inside — that's a coverseam. It's cleaner than a basic hem, more elastic than a lockstitch, and completely flat against the skin.
The technical name comes from the way it works: two needles stitch from the top while a looper "covers" the raw edge from underneath, encasing it completely. No raw edge is visible or exposed. The result is a seam that is as clean on the inside as it is on the outside.
Use the coverseam (ISO 406) for hems and cuff openings. Use the overlock (ISO 514) for joining panels. They work together — overlock closes the garment, coverseam finishes the edges.
Where it appears in a t-shirt
Production specs for your tech pack
| Spec | Value | Notes |
|---|---|---|
| Seam type | Coverseam | ISO 406 classification |
| Needles | 2 (double needle) | Parallel rows visible on outside |
| Stitch density | 10–12 SPI | Same as overlock for consistency |
| Thread | Polyester Tex 18 | Finer thread for low-profile finish |
| Hem allowance | 2 cm | Standard for jersey t-shirt hem |
| Needle | Ball Point SES 70/10–80/12 | Required for knit fabrics |
In FlatLabs PRO, the coverseam is automatically added to construction notes for hems and sleeve openings — including hem allowance, thread spec (Tex 18), and ISO 406 reference.
Thread weight matters here
The coverseam uses a finer thread than the overlock — Tex 18 instead of Tex 27. This is intentional. The coverseam sits on the outside of the garment where it's visible and felt against the skin. A finer thread creates a flatter, softer finish. A heavier thread would create a raised ridge that's both visible and uncomfortable at the hem.
Make sure your BOM specifies two different thread weights if you're using both seam types: Tex 27 for the overlock seams, Tex 18 for the coverseam hems. It's a common spec omission that can cause QC issues at the factory.