Pure Silk twisted thread Approximately 250 Denier Stronger for tying than traditional flat silks Perfect for Northern Spiders, Clyde style flies, and Salmon flies Durable material Pure silk has been used as a fly tying thread for hundreds of years. Semperfli Pure Silk has been designed to match the colors in Edmonds and Lee's book Brook & River Trouting. This is a twisted silk and can be untwisted to lay perfectly flat. Ideal for salmon patterns, North Country and Clyde style fishing flies.