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Unit Systems

A unit system groups related units together, giving the group a name and an abbreviation, allowing for internationalization. The SI unit system subdivides into SI-base, SI-accepted and SI-derived. Similarly, the CGS unit system contains the CGS-EMU system and the CGS ESU system.

The SI-base units are the seven units from which all other SI units can be derived The exact definitions of these units are occasionally updated when more precise and consistent ways to measure them are agreed upon. Generally, each newer definition results in a new value that matches the older one as precisely as was technically achievable at the date of the change.

All SI-derived units are defined in terms of one or more SI base units. E.g. the unit for electrical charge (Coulomb) is defined in terms of the SI-base unit for electrical current (Ampere) and the SI-base unit for time (second). An electical current of 1 Ampere transfers one Coulomb of electrical charge every second. Mathematically: 1 C = 1 A * 1 s. Any change in the definition of one of the underlying base units (i.c. Ampere or second) affects the value of such a derived unit to ensure that the equation is true in every version of the SI system.

The SI-accepted units are conveniance units that are commonly used. E.g. minute, hour and day for time, liter for volume, electronvolt for energy, degree, (arc‑)minute and (arc‑)second for angles.

The CGS-EMU system extends the CGS (Centimeter-Gram-Second) system with units for electromagnetic quantities. The CGS-ESU system extends the CGS system with units for electrostatic quantities. See Centimetre–gram–second system of units.