Super fine tying thread- 2 strands of 0.04mm (2 x 30 Den) thread are wrapped on the bobbin together but not twisted together, combined these are very strong for the amount of thread laid down on the hook so superb for small flies. 

If one of the strands is used alone it can be used for flies as small as size 32 ! 

As the threads are separate it is easy to spin dubbing between them, a single thread is a good alternative to spiderweb for tying the wing post of a Klinkhammer, Hans style.