Coverstitch

Coverstitch machines run two or three needles plus a looper thread that forms a zigzag chain on the underside of the fabric, locking two or three straight rows onto the face of the hem. Unlike a plain lockstitch, that looped underside gives the seam real stretch, so the coverstitch hem can flex with the knit fabric around it instead of popping when it's pulled.

That's the stitch you're actually looking at on the sleeve hems and bottom hem of most cotton and cotton-blend tees, hoodies, and leggings; what shoppers often call "double-needle stitching" from the outside is frequently a coverstitch underneath. On stretch fabrics like a tri-blend tee or an athletic performance knit, coverstitch is close to mandatory, since a rigid straight stitch would tear out the first time someone reaches overhead.

Pros spec coverstitch on any knit with meaningful stretch, rib-knit collars, fleece cuffs, athletic tees, and reserve simple straight double-needle for lower-stretch wovens. Cheap coverstitch machines run loose tension and skip stitches on thick seams, so a quick tug test on a hem before a big order catches a bad setup early.

The common mix-up is treating coverstitch and double-needle stitching as interchangeable terms for the same thing. They look alike from the top, but coverstitch is built on a completely different machine platform, specifically because knit fabric needs a seam that gives.