The Metrics International System of Units

The International System of Units (SI) or (Système International d'Unités) is a modernized version of the metric system established by international agreement. The metric system of measurement was developed during the French Revolution and was first promoted in the U.S. by Thomas Jefferson. Its use was legalized in the U.S. in 1866. In 1902, proposed congressional legislation requiring the U.S. Government to use the metric system exclusively was defeated by a single vote.

The International System of Units was adopted by the 11th General Conference on Weights and Measures (CGPM) in 1960 and is built on the seven SI base units, which are used to define various SI derived units.

The metric system can legally be used in every country in the world (including the United States), and in many countries its use is obligatory. Those countries that still give official recognition to non-SI units (e.g. US, UK) also define those units in terms of SI units.