MonoPoly is a polyester monofilament thread. Commonly called an ‘invisible thread’ due to its fineness (.1 millimeters in diameter - 90 denier). Invisible thread is often used for invisible applique when the stitching is not meant to be seen. MonoPoly is iron and dryer safe (medium heat). It will not become brittle over time nor will it discolor. This thread is strong with low stretch yet it is soft and pliable too!10,000 yds.