From the National Register narrative:
"Pittsburgh's William Penn Hotel is significant as the setting of the city's business and social life; its central position is represented by its location at Sixth and Grant streets among the coporate headquarters and banks founded by the Mellons, Fricks, Pitcairns and Carnegies. The hotel itself is a landmark design by the local firm of Janssen and Abbott, for Henry Clay Frick one of the titans of industry at the height of Pittsburgh's turn of the century boom. Splendid lobbies, elegant dining rooms, a handsome skyline ballroom, and the spectacular art deco room designed by New York theater designer Joseph Urban in the 1928-29 addition form an ensemble unified by consistant classical ornament, in a largeness of scale and a richness of detail that characterizes Pittsburgh architecture. Clearly the largest hotel between New York and Chicago was also intended to be the classiest in the near midwest as well. The result is one of America's great hotels, and one which survives with impressive integrity to describe the ambitions of Pittsburgh's industrial elite.
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The William Penn Hotel occupies an entire block of downtown Pittsburgh facing on of the city's handsomest public spaces, Mellon Square on the and one of the city's great avenues, Grant Street, on the east. The hotel is the product of two phases, the larger or western portion being erected in 1916, and the Grant Street side being added a decade later, in the same palette of materials -- limestone base, deep red brick rising walls and cream colored terra cotta cornice and trim at the top stories. Changes in detail from the nearly Georgian Classicism of the earlier portions to the Mediterranean style of the Grant Avenue facade make the addition apparent --- but not obvious. Interiors are in keeping with the public scale and purpose of the building, and despite changes in colors and carpets, are essentially intact, and rival the monumentality of the exterior.
The range of rooms across the Mellon Square front --- the Georgian Dining Room on the south, the vast wood coffered ceilinged lobby in the center, and the walnut renaissance Terrace Dining Room on the north --- are unified by detail and motif, and again like the exterior contrast with the still traditional but more modern Grant Street lobby. The existing lobby made anything more on Grant Street superfluous, but it is impressive enough with the hexagonal coffered ceiling and mezzanine providing access to the various club and meeting rooms. The middle range of sixteen stories are devoted to the guest rooms; above, the hotel, like Philadelphia's Bellevue Stratford, was crowned by a story of great public rooms offering spectacular views toward the golden triangle of the city. Of these rooms, the grand ballroom (1916) and the black glass Art Deco ballroom by famed New York theater designer, Joseph Urban, are most complete --- and the most memorable.
The Mellon Square facade develops the principal themes of the hotel --- in scale, richness of color, and delicacy of detail. The lower three stories sheathed in limestone, and occupying the entire site, contain the major public spaces. Oversized windows, alternately rectangular- and round-headed on the end pavilion, marking the position of the Terrace and Georgian Rooms, and rectangular opening in the middle, marking the lobby, describe the organization of the building. Elaborately carved window surrounds, belt courses, and a full cornice at the top of the base give the building an urbane richness altogether appropriate for its location and function.
Above, the building rises in three immense towers of deep red brick, separated by deep light wells, to provide air and light to the guest rooms, and joined at the rear by the elevator and service wing. Subtle changes of plane in the brick wall corresponds to the fenestrations of each tower --- paired windows flanked by single windows on the end wings and with the evenly spaced registers of windows on the central tower, which in turn are repeated in the fenestration of the sixteenth floor below the round arched windows of the great public rooms of the seventeenth floor. A massive modillioned cornice caps the facade and continues around on the Sixth and Oliver Avenue facades.