Designed for tying monster-sized flies for fresh- or saltwater, UNI Big Fly Thread is made of continuous polyester filaments. Although it's 4 times as thick, and 4 times as strong, as UNI's 6/0 thread, Big Fly is surprisingly easy to work with; it untwists easily and lays down tight and flat as needed, yet despite its strength it will not dull your fly tying scissors. 400 denier. For tying monster-sized flies Made of continuous polyester filaments 4 times the thickness and strength of 6/0 thread Surprisingly easy to work with Will not dull your tying scissors 400 denier