A superfine tying thread. It has 2 strands of 0.05mm (2 x 30 Den) mono thread that are wrapped on the bobbin together but not twisted together. When combined these are very strong for the amount of thread laid down on the hook for super small flies. Take a needle and split the threads to make dubbing loops and easily spin dubbing/cdc fibers between them. A single thread strand is very similar to spiderweb thread (17/0) for tying the wing post of a klinkhammer and parachute posts.
*100 Yards per spool