Veevus GSP (Gel Spun Poly) Fly Tying Thread! It is available in 5 sizes; the worlds thinnest 30 Denier (Veevus GSP (Gel Spun Poly) Fly Tying Thread- 30 Denier, equivalent to 18/0), 50 Denier (Veevus GSP (Gel Spun Poly) Fly Tying Thread- 50 Denier), 100 Denier, 150 Denier, and 200 Denier. GSP (Gel Spun Polyethylene) Thread is by far the strongest thread on the planet. The Veevus G.S.P. thread has been designed with a slight twist preventing it from opening up, and thereby making tying with it a breeze. It lays completely flat and is the smoothest thread to spin hair with. It is currently available in 2 colors Black & White, with more colors coming soon! 75 meters per spool (which is 67% more thread than offered by anyone else in the industry).