Metrology: what's that? -------------------------
The system of units in use today is the International System of Units (SI). Adopted by a majority of countries in 1960, it comprises seven base units: the metre, kilogram, second, ampere, candela, kelvin, and mole, the last of which was adopted in 1971.
The International System is the successor to the decimal metric system, developed in France in the 18th century to unify the systems then in use. These were often regional and had many drawbacks, chief among them their complexity in practical use.
The unit of length (the metre) was defined as a fraction of the Earth's meridian. The unit of mass (the kilo) was derived from a cubic decimetre of distilled water, making it dependent on the metre. Physical standards were then produced so that these units could be used in practice.