“Nano Silk Pro 20D Black is designed for micro flies down to size 32, however do not be fooled, Nano Silk Pro 20D Black is as strong as 8/0 polyester thread yet is less than 1/20th of the diameter. As a result it can be used for any fly where you do not want to see your thread.” - Semperfli

I use this for all nymphs under a size 18 and especially for dry patterns to keep the body extremely thin.