Often both folding directions are used to produce the desired grain pattern.[7] This process, called the shita-kitae, is repeated from 8 to as many as 16 times. After 20 foldings, (220, or 1,048,576 individual layers), there is too much diffusion in the carbon content. The steel becomes almost homogeneous in this respect, and the act of folding no longer gives any benefit to the steel.[14] Depending on the amount of carbon introduced, this process forms either the very hard steel for the edge (hagane), or the slightly less hardenable spring steel (kawagane) which is often used for the sides and the back.[7]