Well, in the bundle bows there can be a couple kinds of string. Bow string, and lashing string. Just about anything will do for lashing. I used WH Smith's parcel string :roll: only problem is that it does come loose over time and needs relashing, its hard to wax.
Bow string is also pretty easy. I reckon that buying a 1/4 lb roll of B50 Dacron from an archery shop, Quick's isn't the only one, is the easiest. You get enough for a lot of bows, its ready waxed so it sticks easily and is strong enough that you don't have to reverse twist the whole thing. 12-14 strands of dacron will work great for a 50lb bow. If you go below 12 the string starts to get a bit thin, not so weak that it will break on a 40lb bow, but you need more packing so the arrow stays nocked
I wouldn't bother with the linen bow string material that Quick's sells, I bought some and it isn't really all that good. I did a break test and to get a string of 200lb strength (4 x bow weight) made the string too thick to fit standard arrows :yikes:

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If you want to keep it all natural you would do better finding some cobblers type linen thread, say 3 to 7 strand. I got some 3 strand and a doubled length of it (no knots) supported a 35lb dumb bell. Plenty strong enough for a bow string of useful dimensions. More hassle to work though than the dacron because it isn't ready waxed and needs full reverse twisting.
Whatever, you also want some thread for serving the arrow nocking area. Archery shops sell thread for this. You don't need a jig though, just the thread. The cobbler's thread will work for serving too.
You could use the same dacron and serving to make your own cross bow string, just make sure that you use enough strands!
Bow making is addictive, especially since it needs LOTS fewer tools than knife making!
Bit sad that I could remember that Bowyer's Bible reference without looking :rolmao: