Bobbin thread

Bobbin thread runs on the underside of the garment, feeding from a small spool housed in the machine's bobbin case, and it interlocks with the top (needle) thread at every single stitch to form a lock stitch. It's typically a thinner, plain white, black, or gray polyester or cotton thread — most shops stock just one or two bobbin colors and use them across every job regardless of what the face of the design looks like.

Flip over any embroidered logo and you'll see this immediately: the front might carry six colors in a detailed crest, but the back shows a flat, single-color mesh of stitching that looks nothing like the design. Customers occasionally call this in as a defect — "the back doesn't match the front" — but that's simply how lock-stitch embroidery works on every commercial machine, not a shop-specific shortcut.

Pros choose bobbin color mainly for how it reads through thin or light-colored fabric; a white bobbin thread can show through as tiny dots on the inside of a dark, lightweight polo, so some shops keep a black bobbin thread on hand specifically for dark garments to keep the backside from looking speckled. Bobbin tension also gets checked as part of routine machine maintenance, since a too-loose or too-tight bobbin causes visible loops or puckering on the top side.

The common mistake is assuming bobbin thread should match the top thread color-for-color — it doesn't need to and, on most production runs, never will. What actually matters is correct tension and a bobbin color that won't show through the fabric from the front.