Durometers are expressed using a number and a letter scale. While the specific scale used changes based on the circumstances of the hardness measurement, generally a higher number means a harder and more durable material, while a lower number means a softer and more flexible material. Each numerical rating is then paired with an alpha scale to express what range of hardness is being measured. The main two hardness scales used are the Shore hardness scale and the Rockwell hardness scale. Rockwell scales are often used with harder material measurements, such as nylon or polycarbonate, while the Shore hardness scales are used with rubbers, softer plastics, and elastomers. The common alpha scales for Rockwell durometer are Rockwell E, Rockwell M, and Rockwell R. Whereas the major alpha scales for Shore hardness include Shore OO, Shore A, and Shore D. Which scale is used depends on the material being measured as well as the industry you work in.