The building was part of the later Rockefeller Center expansion (1960s–1970s) dubbed the "XYZ Buildings". Their plans were first drawn in 1963 by the Rockefeller family's architect, Wallace Harrison, of the architectural firm Harrison & Abramovitz. Their letters correspond to their height. 1251 Avenue of the Americas is the "X" Building as it is the tallest at 750 ft (229 m) and 54 stories, and was the first completed, in 1971. The "Y" is 1221 Avenue of the Americas, which was the second tower completed (1973) and is the second in height (674 ft and 51 stories). The "Z" Building, the shortest and the youngest, is 1211 Avenue of the Americas with 45 stories (592 ft). 1251 is the second-tallest building in the whole of Rockefeller Center, after 30 Rockefeller Plaza.