A simple way to think of San Mai III blade construction is to imagine a sandwich: The meat center is hard, high carbon steel and the pieces of bread on either side are the lower-carbon, tough side panels. The edge of the blade should be hard to maximize edge holding ability, but if the entire blade was hard it could be damaged during the rigors of battle. For ultimate toughness the body of the blade must be able to withstand impact and lateral stresses. Toughness is generally associated with "softness" and "flexibility" in steel, so that, surprisingly, if a blade is made "tough" the edge won't be hard enough to offer superior edge holding. San Mai III® blades provides a blade with hard (higher carbon) steel in the middle for a keen, long lasting edge and tougher (lower-carbon) steel along the sides for flexibility.