Fil-Tec Magna-Quilt Prewound Bobbins Egyptian Long Staple Mercerized Cotton No. 50/2 (Tex 24). Has the look of cotton but has the strength and colorfastness of polyester. The magnetic core rotates the bobbin consistently throughout the entire bobbin which results in uniform tension to the last stitch. Consistent tension throughout the bobbin increases stitches per bobbin. This equates to more time quilting and fewer bobbin changes. No backlash or overspin issue.