of metal called . The function of the softer base metal is to absorb shock, and to protect the more brittle ha-gane from breaking. This technology allows for the use of steels in the hagane which are harder than in use in Western chisels, typically Rockwell 62 and up, and also allows for the honing of a much finer edge than is typically known in carpentry outside Japan. When sharpening a blade, a Japanese carpenter will typically use three or more whetstones of varying coarseness, progressing from the roughest stone to the finest. The blades of both planes and chisels are distinguished by the hollow, ura in their flat side. This hollow portion has a number of functions. The primary function is that it ensures a high degree of flatness when sharpening, in that when the flat side is polished it cannot rock or develop a curve because it is only contacting the stone on either side of its width. This then improves the precision with which cuts can be made by the chisel, and in the case of planes ensures smooth contact with the wedge and therefore even support across the full width of its blade. The hollow also greatly reduces the amount of metal needed to be removed to achieve flatness on the back of the blade, which shortens initial set-up and subsequent re-sharpening considerably. Secondly, in the case of chisels, it reduces the frictional resistance as the chisel is driven into or extracted from the wood. Thirdly, the interaction of the leading edge of the hollow with the edge of the blade is a changing relationship as the tool is re-sharpened. With plane blades, as the edge is sharpened down to the rim of the hollow, the edge can then be 'tapped-out' (ura-dashi), a process where a pointed hammer is used to depress the ha-gane downward slightly along the bevel of the blade. When the blade's back is re-flattened after ura-dashi, the hollow is re-established; thus the hollow acts as a sort of gauge for sharpening as a means of prolonging the life of the thin piece of cutting steel as long as possible. This in turn tends to keep the geometry of the blade consistent over time, which keeps it fitting the dai over time. There are many types of steel used for the ha-gane of Japanese planes and chisels: White steel, shiro-gane; a nearly pure