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. Longbows, because of their narrow limbs and rounded cross-section (which does not spread out stress within the wood as evenly as a flatbow’s rectangular cross section), need to be less powerful, longer or of more elastic wood than an equivalent flatbow. In Europe the last approach was used, with yew being the wood of choice, because of its high compressive strength, light weight, and elasticity. Yew is the best widespread European timber that will make good self longbows, (other woods such as elm can make longbows but require heat-treating of the belly and a wider belly/narrower back, while still falling into the definition of a longbow) and has been the main wood used in European bows since Neolithic times. More common and cheaper hard woods, including elm, oak, hickory, ash, hazel and maple, are good for flatbows. A narrow longbow with high draw-weight can be made from these woods, but it is likely to take a permanent bend (known as "set" or "following the string") and would probably be outshot by an equivalent made of yew. Wooden laminated longbows can be made by gluing together two or more different pieces of wood. Usually this is done to take advantage of the inherent properties of different woods: some woods can better withstand compression while others are better at withstanding tension. Examples include hickory and lemonwood, or bamboo and yew longbows: hickory or bamboo is used on the back of the bow (the part facing away from the archer when shooting) and so is in tension, while the belly (the part facing the archer when shooting) is made of lemonwood or yew and undergoes compression (see bending for a further explanation of stresses in a bending beam). Traditionally made Japanese