Ultra GSP is the strongest thread on the planet. Boasting steel like strength for its weight, this thread really lets you put the pressure on! The gel spun polyethylene is supple, ties flat and is available in the largest selection of sizes and colors. Choose 50 denier GSP for tying small trout flies with a minimum of bulk. GSP 75 and 100 has all around versatility for fresh and saltwater flies and GSP 200 is the size to choose for big deer hair work or when you really need to crank down!