The International System of Units is a consistent system of units adopted and recommended by the General Conference on Weights and Measures. In the International System of Units, units are divided into three categories: basic units, derived units and auxiliary units. The seven strictly defined basic units are: length (meter), mass (kilogram), time (second), electric current (ampere), thermodynamic temperature (kelvin), amount of substance (mole) and luminous intensity (candela). The basic units are dimensionally independent of each other. There are many derived units, all of which are composed of basic units. There are only two auxiliary units, which are purely geometric units. Of course, auxiliary units can also constitute derived units. Various physical quantities are related to each other through equations that describe natural laws and their definitions. For convenience, a group of independent physical quantities are selected as basic quantities, and other quantities are expressed based on basic quantities and related equations, which are called derived quantities.