aspect common to mutually comparable quantities

Note 1 to entry: The division of the concept of quantity into several kinds of quantity is to some extent arbitrary. Examples:

- The quantities diameter, circumference, and wavelength, are generally considered to be quantities of the same kind, namely of the kind of quantity called length.
- The quantities heat, kinetic energy, and potential energy, are generally considered to be quantities of the same kind, namely of the kind of quantity called energy.

Note 2 to entry: Quantities of the same kind within a given system of quantities have the same dimension of a quantity. However, quantities of the same dimension are not necessarily of the same kind.

Examples: The quantities moment of force and energy are not of the same kind, although they have the same dimension. Similarly for heat capacity and entropy, as well as for relative permeability and mass fraction.

Note 3 to entry: The term "kind" is mainly used in expressions such as "quantities of the same kind." Two quantities of the same kind are mutually comparable, so that they can be placed in order of magnitude. Length and mass are quantities of different kinds because they are not mutually comparable.

[SOURCE: ISO/IEC Guide 99:2007, 1.2, modified – Note 3 has been modified.]