YZD Ultra thread 135 deniers is a good general purpose fly tying thread that's perfect for tying flies in the #6 to #14 size range.This kind of thread is called polyester filament, not a multi-strand synthetic thread. The characteristic is very slim, very suitable for making small and ultra-small Fies, large size fly can also be competent,but the strength is slightly worse than the multi-strand synthetic thread. Please be sure to master the pulling force when using.