Two kinds of artificial sinew, one is nylon and one is polyester. The polyester is very similar to B-50, the nylon stretches to much to make it useful. I like fastflight for first brace on a self bow because it doesn't stretch and you don't have to bend the bow so far to get it braced at the right height. I have heard that fishing tackle stores sell "spider wire" which is supposed to be the same thing as fastflight.