The east and center wings were designed by Robert Mills, architect of the Washington Monument. These wings comprise the oldest part of the building, constructed from 1836 to 1842. The most architecturally impressive feature of the Mills design is the colonnade running the length of the building. Each of the 30 columns is 36 feet tall and each was carved out of a single piece of granite. The interior design of the east and center wings is classically austere, in keeping with the Greek Revival style.