Ultra GSP is the strongest thread on the planet! Boasting a steel-like strength for its weight, this thread really lets you put the pressure on! Our gel spun polyethylene is supple and ties flat. Choose 50 GSP for tying small trout flies with a minimum of bulk. GSP 75 and 100 have all-around versatility for fresh and saltwater flies and GSP 130 or 200 are the sizes to choose for big deer hair work or when you really need to crank down!