Monofilament thread, often called clear or invisible thread, is used to show off the quilting but not the thread. Most monofilament thread is made of nylon, not polyester, and is the same substance used in fishing line. Superior MonoPoly is made of polyester, not nylon. The advantages of polyester monofilament are numerous: Polyester has a much higher heat resistence than nylon. Polyester monofilament does not go brittle over time. Polyester monofilament does not discolor or "yellow" over time. Polyester is softer and more pliable than nylon, meaning it will lie better on and in the fabric. Thread size .004 Spool size: 2,400 yds.