SOS is a brand of mono produced by Leland's Lures. It comes in 2, 4, and 6 pound test. I use it for my tippet material too. It seems very strong and is limp. The SOS stands for: S strong O obscure S small. It comes on a spool containing 350 yards for around $5. This is the same company that produces the Trout Magnet lure that spinner fishermen use.