Gusset

Cut a diamond of fabric two or three inches across, set it into a stress point in the seam, and you've got a gusset — the reinforcement mills use at underarms and crotches to buy the wearer extra range of motion without tearing the seam itself. On heavyweight work shirts like a Carhartt K87, that underarm panel is usually matching twill turned on point; on athletic and cycling gear it's often a stretch mesh or spandex-blend insert instead of the shirt's main fabric, since the goal there is movement, not just reinforcement.

This kind of reinforcement is one of those details that separates a $9 promotional tee from an $18 workwear or athletic blank — it's extra cutting and sewing labor mills only bother with when a garment is built to be reached overhead, squatted in, or worked in all day. We've watched a gym order get returned wholesale after a single class because the blank's underarm wasn't built this way and blew out on the first overhead reach.

If your buyer is a mechanic, a nurse, a CrossFit gym, or anyone ordering uniforms that get worn hard, ask specifically whether the blank is gusseted before you quote the job — it won't show up on a spec sheet's chest or body measurements, only in the construction notes. That extra panel also eats into the lower few inches of sleeve real estate, which changes where a sleeve print can safely sit.

People often assume any reinforced-looking underarm stitching qualifies as a gusset; it doesn't, unless there's an actual inserted panel of fabric, not just a bar tack or an extra row of stitching. A double-stitched underarm without one is still just a flat seam — sturdier, but it won't give you the range of motion a true gusset provides.