For decades, the diameter of fly tying thread was listed in “aught” sizes: 8/0, 6/0, and 3/0. Size A was also once a common fly tying thread; it was very thick, suitable for making very large streamers and saltwater patterns. Now the market is over saturated and trying to decide on sizes more difficult. The Essential Fly.com site has put up a fairly “direct to the point” article on choosing the right thread. Check it out!