By convention every 2D roughness parameter is a capital formula_13 followed by additional characters in the subscript. The subscript identifies the formula that was used, and the formula_13 means that the formula was applied to a 2D roughness profile. Different capital letters imply that the formula was applied to a different profile. For example, formula_1 is the arithmetic average of the roughness profile, formula_16 is the arithmetic average of the unfiltered raw profile, and formula_3 is the arithmetic average of the 3D roughness.