Flatlock Seam

A flatlock seam is built by overlapping two fabric edges, rather than folding them under, and stitching them together so the seam presses completely flat against the body, with the interlocking loops forming a decorative ladder-stitch look on the outside. Because there's no folded seam allowance standing proud, a flatlock seam has essentially zero ridge left to rub against skin during movement.

This is the seam down the sides and sleeves of compression shirts, running singlets, and a lot of Sport-Tek and Badger performance tees, wherever a garment is going to see sweat, friction, and repetitive motion. We've had athletic-apparel buyers specifically request flatlock construction after a batch of standard side-seamed shirts came back with complaints about chafing on long runs.

Spec flatlock seams for anything marketed as performance, athletic, or compression wear, especially in moisture-wicking polyester or spandex blends where friction and sweat make a raised seam noticeable fast. It costs more per garment than a standard seam, so it tends to show up selectively at the side seams and sleeve seams rather than everywhere on the shirt.

A common mistake is assuming flatlock seams are just a style choice for the ladder-stitch look. The flat profile is functional first; the visible stitching is a byproduct of a construction method chosen to eliminate chafing, not a decorative flourish added afterward.