It was eventually realized that TDB was not well defined because it was not accompanied by a general relativistic metric and because the exact relationship between TDB and TDT had not been specified. (It was also later criticized as being not physically possible in exact accordance with its original definition: among other things the 1976 definition excluded a necessary small offset for the initial epoch of 1977.) After the difficulties were appreciated, in 1991 the IAU refined the official definitions of timescales by creating additional new time scales: Barycentric Coordinate Time (TCB) and Geocentric Coordinate Time (TCG). TCB was intended as a replacement for TDB, and TCG was its equivalent for use in near-Earth space. TDT was also renamed to Terrestrial Time (TT), because of doubts raised about the appropriateness of the word "dynamical" in that connection.