The main part of the Cathedral's first floor is the Commons Room, called one of the "great architectural fantasies of the twentieth century," is a Gothic-style hall that covers half an acre and extends upwards four stories, reaching 52 feet tall. The room was a gift of Andrew Mellon. It is a piece of true Gothic architecture; no steel supports were used in the construction of its arches. Each arch is a true arch, and they support their own weight. Each base for the arches weighs five tons, and it is said that they are so firmly placed that each could hold a large truck. The large central piers act only as screens for the structural steel that holds up the upper floors of the building. Despite its heavy use, the Commons Room is kept quiet by the use of Guastavino acoustical tiles as the stones between the ribs of vaulting.