SI units: one system, three kinds

Before 1960, a “pound” in Lahore was not quite a “pound” in London, and a foot was whatever length a local ruler happened to use. Trade, science and exams all suffered from the same confusion: units that meant slightly different things in different places.

In 1960, an international committee finally agreed on one shared set of standards for every physical quantity: the System International, or SI. It is built from three kinds of units working together.

Seven base units cover the seven base quantities. Two supplementary units cover angles. Every other unit in physics, however exotic it looks, is derived by combining these nine.