The metric system of measurement is an internationally agreed-upon set of units for expressing the amounts of various quantities such as length, mass, time, temperature, and so on.

Whenever we measure something, from the weight of a sack of potatoes to the distance to the moon, we must express the result as a number of specific units: for example, pounds and miles in the English system of measurement (although even England no longer uses that system), or kilograms and kilometers in the metric system. As of 1994, every nation in the world has adopted the metric system, with only four exceptions: the United States, Brunei, Burma and Yemen.

The metric system that is in common use around the world is only a portion of the broader International System of Units, a comprehensive set of measuring units for almost every measurable physical quantity from the ordinary, such as...