Math & science · 1960

International System of Units

The SI unifies measurement worldwide around seven base units.

Adopted by the General Conference on Weights and Measures, it redefined the second by caesium in 1967 and the metre by light in 1983. Shared units make global trade and science interoperable.

See it in the living tree Wikipedia

Arcs above join it to what it stood on, arcs below to what it led to. The bar places this stretch on the tree's whole timeline.

What it built on 2

Atomic clock 1955
Metric system 1795

What it led to 1

SI redefined by constants 2019