40 wt, Style M, set of 10 bobbins

Filament Polyester . Fil-Tec Bobbin central's patented magnetic core creates consistent delivery through the entire bobbin. No more interruptions for tension adjustments as your bobbin gets smaller. Magnetic core prevents backlash or over-spin and eliminates the need for backlash springs. Magna-Glide is the original high-yield magnetic-core bobbins.