what is a system of units

A system of units of measurement is a collection of units of measurement and rules relating them to each other. It is used to measure physical quantities such as length, mass, time, and temperature. The International System of Units (SI) is the modern form of the metric system and the worlds most widely used system of measurement. It is a decimal and metric system of units established in 1960 and periodically updated since then. The SI has an official status in most countries, including the United States, Canada, and the United Kingdom, although these three countries are amongst a handful of nations that, to various degrees, also continue to use their customary systems. The SI consists of a set of SI base units, SI derived units, and a set of decimal-based multipliers and submultipliers that are used as SI prefixes. The units, excluding prefixed units, form a coherent system of units, which is based on a system of quantities in such a way that the equations between the numerical values expressed in coherent units have exactly the same form, including numerical factors, as the corresponding equations between the quantities themselves.