One of the simpler longbow designs is known as the self bow, by definition made from a single piece of wood. Traditional English longbows are self bows made from yew wood. The bowstave is cut from the radius of the tree so that sapwood (on the outside of the tree) becomes the back and forms about one third of the total thickness; the remaining two thirds or so is heartwood (50/50 is about the maximum sapwood/heartwood ratio generally used). Yew sapwood is good only in tension, while the heartwood is good in compression. However, compromises must be made when making a yew longbow, as it is difficult to find perfect unblemished yew. The demand for yew bowstaves was such that by the late 16th century mature yew trees were almost extinct in northern Europe. In other desirable woods such as Osage orange and mulberry the sapwood is almost useless and is normally removed entirely.