(For a more accessible and less technical introduction to this topic, see Introduction to the metric system.) The metric system is an internationally agreed decimal system of measurement. It was originally based on the mètre des Archives and the kilogramme des Archives introduced by the First French Republic in 1799, but over the years the definitions of the metre and the kilogram have been refined, and the metric system has been extended to incorporate many more units. Although a number of variants of the metric system emerged in the late nineteenth and early twentieth centuries, the term is now often used as a synonym for "SI" or the "International System of Units"—the official system of measurement in almost every country in the world.