“Twin needle employs a chain stitch, single needle a lock stitch (conventional top and bobbin thread). Look at the diagrams here: http://home.howstuffworks.com/sewing-machine.htm Have a look at the inside leg seam of a pair of Levis. That is a twin needle seam. Both fabric edges are folded over and then a twin needle machine, utilizing a chain stitch goes over the lot. Do you feel the roughness of the underside? That is where the thread forms a loop. A single needle seam is equally stitched twice but with a flat felled seam. One row of stitching is hidden inside the fabric. From the topside you see only one row, but you see two rows of stitching from the underside. http://www.backpacking.net/makegear/…latfelled.html Twin needle seams are less labor intensive, but more prone to puckering. Most American shirts are single, most European ones, bar the top makes, are twin needle. ”
Double Needle Tailoring
The difference is explained by the durability. For chainstitching, if one joint breaks, the entire seam will break as it is a “Chain”. However, a lockstitch is able to stay intact even if you break a link.
from putthison.com in a discussion about quality of men’s shirts…to evaluate custom-made shirts…
“Stitches per inch: Low-end shirts tend to be made with fewer stitches-per-inch than high-end shirts. That’s because the speed of a sewing machine is measured in stitches-per-minute. The fewer stitches a machine has to sew per inch, the faster it can go. Since poorly made shirts are banged out as fast as possible, they have lower stitch counts. These rougher looking stitches detract from the shirt’s durability and elegance. ”
I’ve recently become acquainted with Do You Sew – www.doyousew.com and feel we have much the same philosophy. You might want to check out this blogpost of theirs: