text: 1 scale on 13 September 1956, using an Atomichron commercial atomic clock, followed by the NBS-A scale at the National Bureau of Standards, Boulder, Colorado on 9 October 1957.The International Time Bureau (BIH) began a time scale, Tm or AM, in July 1955, using both local caesium clocks and comparisons to distant clocks using the phase of VLF radio signals.The BIH scale, A.1, and NBS-A were defined by an epoch at the beginning of 1958 The procedures used by the BIH evolved, and the name for the time scale changed: A3 in 1964 and TA(BIH) in 1969.The SI second was defined in terms of the caesium atom in 1967.From 1971 to 1975 the General Conference on Weights and Measures and the International Committee for Weights and Measures made a series of decisions that designated the BIPM time scale International Atomic Time (TAI).In the 1970s, it became clear that the clocks participating in TAI were ticking at different rates due to gravitational time dilation, and the combined TAI scale, therefore, corresponded to an average of the altitudes of the various clocks.Starting from the Julian Date 2443144.5 (1 January 1977 00:00:00), corrections were applied to the output of all participating clocks, so that TAI would correspond to proper time at the geoid (mean sea level).Because the clocks were, on average, well above sea level, this meant that TAI slowed by about one part in a trillion.The former uncorrected time scale continues to be published under the name EAL (Échelle Atomique Libre, meaning Free Atomic Scale).The instant that the gravitational correction started to be applied serves as the epoch for Barycentric Coordinate Time (TCB), Geocentric Coordinate Time (TCG), and Terrestrial Time (TT), which represent three fundamental time scales in the solar system.All three of these time scales were defined to read JD 2443144.5003725 (1 January 1977 00:00:32.184) exactly at that instant.