The International System of Units (SI, Système international (d'unités)) is the modern form of the metric system, and is the most widely used system of measurement. It's a coherent system of units of measurement built on seven base units from nature, which are the ampere, kelvin, second, meter, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system also specifies names for 22 derived units, such as lumen and watt, for other common physical quantities.
The base units are derived from INVARIANT CONSTANTS OF NATURE, such as the speed of light in vacuum and the triple point of water, which can be observed and measured with great accuracy, and just one physical artifact. But that is changing. The artifact is the international prototype kilogram, certified in 1889, and consisting of a cylinder of platinum-iridium, which nominally has the same mass as one liter of water at the freezing point. Its stability has been a matter of significant concern, culminating in a revision of the definition of the base units ENTIRELY in terms of constants of nature, and the new Kg is a derivative of the Planck constant and new exact masses can be found everywhere without the original prototype.
The "metric system" is beautiful, exact, easy to handle, easy to calculate, and now independent of a master prototype's held somewhere.
Who would use systems based on body imprecise parts like inches or feet, and weird not decimal fractions of them? That's so medieval.