The Rockwell scale is a hardness scale used to measure the hardness of materials. It is based on the indentation hardness principle, which states that the depth of penetration of an indenter into a material is a function of the applied load and the properties of the material such as its elastic modulus, yield strength, and hardness. The Rockwell scale consists of several different scales, each denoted by a letter, that use different loads or indenters. Each scale produces a numerical value that corresponds to a particular level of hardness. The most commonly used scales are A, B, C, and D.