I agree for the most part with Donald but I do use other threads to tie soft hackles. Silk "usually" comes in a small diameter that makes it ideal for small soft hackles and dries although some other modern threads can be had in small diameters also.

I mostly use white silk thread for reinforcing quill body flies by criss-crossing it over the quill body and then adding a drop of Spar varnish. The silk seems to disappear, and the varnish also helps to reinforce the fragile quill body.

Robert B. McCorquodale
Sebring, FL

"Flip a fly"