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 an indentation in a material is proportional to the load applied to create it. The Rockwell scale is divided into several different scales, each with its own range and values. The most commonly used scales are the A, B, C, and D scales. Each scale measures a different range of hardness; for example, the A scale measures very soft materials such as aluminum, while the D scale measures very hard materials such as steel.