units that don't belong to any comprehensive system of units, but that are nevertheless still regularly used in particular fields and regions. Both of these categories of unit are also typically defined legally in terms of SI units. Controlling body The SI was established and is maintained by the General Conference on Weights and Measures (CGPM). In practice, the CGPM follows the recommendations of the Consultative Committee for Units (CCU), which is the actual body conducting technical deliberations concerning new scientific and technological developments related to the definition of units and the SI. The CCU reports to the International Committee for Weights and Measures (CIPM), which, in turn, reports to the CGPM. See below for more details. All the decisions and recommendations concerning units are collected in a brochure called The International System of Units (SI), which is published by the International Bureau of Weights and Measures (BIPM) and periodically updated. Overview of the units SI base units The SI selects seven units to serve as base units, corresponding to seven base physical quantities. They are the second, with the symbol , which is the SI unit of the physical quantity of time; the metre, symbol , the SI unit of length; kilogram (, the unit of mass); ampere (, electric current); kelvin (, thermodynamic temperature); mole (, amount of substance); and candela (, luminous intensity). All units in the SI can be expressed in terms of the base units, and the base units serve as a preferred set for expressing or analysing the relationships between units. SI derived units The system allows for an unlimited number of additional units, called derived units, which can always be represented as products of powers of the base units, possibly with a nontrivial numeric multiplier. When that multiplier is one, the unit is called a coherent derived unit. The base and coherent derived units of the SI together form a coherent system of units (the set of coherent SI units). Twenty-two coherent derived units have been provided with special names and symbols. The seven base units and the 22 derived units with special names and symbols may be used in combination to express other derived units, which are adopted to facilitate measurement of diverse quantities. Why SI kept the distinction between base and derived units Prior to its redefinition in 2019, the SI was defined through the seven base units from which the