These were normally made of hemp, gut or silk and either twisted or plaited with
beeswax (for waterproofing) to the desired length. I have heard that steel strings were
sometimes used for some of the middle eastern bows, but have not found references for
this (and would hate to be using one if it snapped during use. The thought of steel wire
under stress snapping close to the check and eye with 50+ pounds of tension on it doesn't
inspire me). Often a loop is placed in one end, and the other end left hanging. When the
bow is strung, this end was tied using a bowyers knot (now called a bowline knot).
Other methods allowed the maker to plait or twist a loop into either end during construction (e.g the Flemish twist method). Turkish strings were made with separate end loops (tundj) tied to the string with a special knot (same with Chinese, Mongolian, Persian and Tatar, probably others as well too) allowing it to be shortened or made longer to fit a particular bow/archer, the loop added stability to these short recurves. Recently I was informed that the researchers on the Tudor ship "Mary Rose" have found their first complete bowstring of the period. It was preserved intact under the cap of it's unfortunate owner. The string itself is a very strong variant of English linen, although whether plaited, woven or endless string I am unsure.
Offered by Brian.