Hardness of materials has probably long been assessed by resistance to scratching or cutting. An example would be material B scratches material C, but not material A. Alternatively, material A scratches material B slightly, and scratches material C heavily.
Relative hardness of minerals can be assessed by reference to the Moh's scale that ranks the ability of materials to resist scratching by another material. Similar methods of relative hardness assessment are still commonly used. An example is the file test where a file is tempered to a desired hardness and is then rubbed on the test material surface.
These relative hardness tests are limited in practical use and do not provide accurate numeric data or scales particularly for modern day metals and materials. The usual method to achieve a hardness value is to measure the depth or area of an indentation left by an indenter of a specific shape with a specific force applied for a specific time.
There are three principal standard test methods for expressing the relationship between hardness and the size of the impression, these being Brinell, Vickers and Rockwell. For practical and calibration reasons, each of these methods is divided into a range of scales, defined by a combination of applied load and indenter geometry.