Editing Metric system (section)

==Base units==

The [[Decimal|decimalised system]] is based on the [[metre]], which had been introduced in [[French Revolution|France in the 1790s]]. The historical development of these systems culminated in the definition of the [[International System of Units]] (SI) in the mid-20th century, under the oversight of an international standards body.

The historical evolution of metric systems has resulted in the recognition of several principles. A set of independent dimensions of nature is selected, in terms of which all natural quantities can be expressed, called base quantities. For each of these dimensions, a representative quantity is defined as a [[base unit (measurement)|base unit]] of measure. The definition of base units has increasingly been [[realisation (metrology)|realised]] in terms of fundamental natural phenomena, in preference to copies of physical artefacts. A unit [[SI derived unit|derived]] from the base units is used for expressing quantities of dimensions that can be derived from the base dimensions of the system—e.g., the square metre is the derived unit for area, which is derived from length. These derived units are [[Coherence (units of measurement)|coherent]], which means that they involve only products of powers of the base units, without any further factors. For any given quantity whose unit has a name and symbol, an extended set of smaller and larger units is defined that are related by factors of powers of ten. The unit of time should be the [[second]]; the unit of [[length]] should be either the metre or a decimal multiple of it; and the unit of mass should be the gram or a decimal multiple of it.

Metric systems have evolved since the 1790s, as science and technology have evolved, in providing a single universal measuring system. Before and in addition to the SI, other metric systems include: the [[MKS system of units]] and the [[Giovanni Giorgi#The Giorgi system|MKSA]] systems, which are the direct forerunners of the SI; the [[Centimetre–gram–second system of units|centimetre–gram–second (CGS)]] system and its subtypes, the [[Centimetre–gram–second system of units#Electrostatic units (ESU)|CGS electrostatic]] (cgs-esu) system, the [[Centimetre–gram–second system of units#Electromagnetic units (EMU)|CGS electromagnetic]] (cgs-emu) system, and their still-popular blend, the [[Gaussian units|Gaussian system]]; the [[metre–tonne–second system of units|metre–tonne–second (MTS)]] system; and the [[gravitational metric system]]s, which can be based on either the metre or the centimetre, and either the gram, gram-force, kilogram or kilogram-force.