Semperfli Floss Fly Tying Thread - Different to other flosses on the market, Semperfli fly tying Floss is thin at just 400 Denier it can be used for smaller trout flies and doubled over for use on Salmon patterns, including classic salmon flies. By creating a thinner fly tying floss the capabilities and uses of this stunning material are enhanced so it can be used as the body for flies of many sizes. There are 27 yards (approx) per spool of Fly Tying Floss. The thickness of the floss is 400D or 2/0 on the Imperial Scale.