The hardness of a plastic, rubber or elastomer is measured by the depth of indentation by a shaped metal foot on a measuring instrument known as a durometer. The depth of indentation is measured on a scale of 0 to 100; higher numbers indicate harder materials. These tests are generally used to denote the flexibility of a material. The 2 most popular are Shore A and Shore D. Indenter feet with different profiles are used in each case, and so each method is suitable for different material harnesses. For example, soft materials are measured on the Shore A scale, and hard materials are measured on the Shore D scale. There is not a strong correlation between different scales.