A type of steel that has enough carbon in it to make it very tough, yet not so tough that it becomes brittle and inflexible. Sword blades made out of high-carbon steel tend to keep an edge well, but be flexible enough that you can bend them over 3" out of line and they will flex back true - if they are tempered properly. Stainless either won't bend or will bend and stay bent, for the most part. Unfortunately, carbon steel rusts easily and you have to keep it oiled. Gun oil works well for this. Note that the temper of a carbonized steel blade will determine how flexible it is - an improperly tempered carbon steel blade will be hard but brittle, and shatter easily.