The Rockwell hardness scale is likely the most common hardness scale used today. A highly accurate and fast method of testing hardness, the Rockwell hardness test is used across a number of metrological settings and so you often see hardness represented using the Rockwell hardness scale. Information collected about hardness relates to other factors held by a material including strength, resistance, and ductility, making precision highly important. The differential depth measurement concept for hardness that is used by the Rockwell hardness scale was first developed by Pail Ludwik in 1908. Then, Hugh Rockwell and Stanley Rockwell co-invented a machine called the Rockwell hardness tester, which was patented in 1914, that implemented the differential depth measurement concept. The Rockwell hardness scale is defined by the standard American Society for Testing and Materials (ASTM) E18 and approved for measurement of commercial shipments, making it highly utilized. Getting a reliable Rockwell hardness scale value requires that the test materials be at least 10 times the depth of the indentation being made and the material must be measured on a flat and perpendicular surface.