Other than a hook it is fly tying thread that it is the most important piece of fly tying material and deserves special selection. Modern fly tying threads are made from GSP (Gel Spun Polyethylene) which are really thin and extrodinarily strong. More traditional fly tying threads are made from polyester and the oldest style tying threads are made from pure silk which is today primarily reserved for ribs and bodies. So what makes a fly tying thread? gsp, polyester, silk or rayon is twisted around itself to make the fly tying thread which can then have one or two strands (or cores). Some threads being 2 core or very lightly twisted such as Classic Waxed Thread or GSP threads are easier to form dubbing loops than some more tightly twisted traditional fly tying threads.