The time-scale that has been used by international agreement since 1972. It is ultimately based upon atomic oscillations, rather than astronomical principles, and is the most accurate time-scale available today. Its fundamental unit is the SI second, defined in terms of properties of the caesium atom (see Atomic Time). TAI differs from Coordinated Universal Time (UTC) by an exact number of seconds, because leap seconds are introduced in UTC to keep it in step with the changeable rotation rate of the Earth. TAI was defined so as to be in agreement with UTI on 1958 January 1. It is maintained by a coordinated network of several hundred atomic clocks located at astronomical observatories and standards laboratories around the world. See also Universal Time.