Wikipedia: 's architect, Wallace Harrison of the architectural firm, Harrison and Abramovitz. Their letters correspond to their height. 1251 is the X Building as it is the tallest 750 ft (229 m) and 54 stories, but was the second one completed . The Y is the McGraw-Hill Building, at 1221 Avenue of the Americas, which was the first completed and is the second in height . The Z Building, the shortest and the youngest, is the Celanese Building at 1211 Avenue of the Americas with 45 stories (592 ft). 1251 is the second-tallest building in the whole of Rockefeller Center, after the GE Building. In 2006, the large pool and fountains facing Sixth Avenue were under renovation.. ExxonMobil Corporation subsequently moved its headquarters to Irving in Texas, and its New York offices to Brooklyn; it no longer retains a presence in Rockefeller Center. Inside, on the western end of 1251's atrium hangs an artist-authorized replica of a tapestry Pablo Picasso created for the ballet Mercure, the original of which hangs in the Musée National d'Art Moderne in Paris, France. It was created specifically for 1251, as per the plaque beneath it.