Magna-Glide Calssic magnetic core bobbins are the only embroidery bobbin specifically designed to sew without tension variation throughout the entire bobbin. The magnetic core, combined with our advanced EB "crosslink" technology optimizes dynamic thread delivery to create consistent tension from beginning to end of each bobbin. Fiber Type: Polyester Fiber Size: Tex 12 The magnetic core provides uniform tension. The bobbin will sew to the very end without tension variation. Also, high yield fiber equals more stitches per bobbin. This is a great bobbin for home machines with a metal bobbin case.~