Bamboo-backed osage orange. This is the fastest combination of materials that I have ever come up with. Following Dean Torges and others, I glued a thin lamination of bamboo to the back of a core of osage orange (bowdark or mock orange or hedgeapple wood). The shape is called "reflex-deflex," indicating the double curve shape of the bow. This particular bow is very fast. I would tell you how fast, but that information is classified!