The feet that hold the thread spool are made with Delrin, so the spool rotates more smoothly than any bobbin you've ever used. These bobbins give you smooth, consistent tension. The tube is flared at both ends, and lined with ultrahard black ceramic to minimize wax buildup and thread wear. Curved bobbin has slightly curved shape that is comfortable in the hand. Magnum bobbin will handle strong threads and large flies with ease-excellent for spinning deer hair or tying large steelhead, salmon, or saltwater flies. Supreme bobbin has straight tube for standard work.