The Gallery, or "Slips"

According to James Mease,

The wings [of the building], opened by large windows, recede a little from front, above, but project below, twelve feet, to the line of the street, faced with marble; these pavilions are decorated with emblematic figures, in tablets, and connected together by a colonnade of ten fancy Corinthian columns. . .Through the[se] projecting wings or pavilions, you pass to the stairs of the galleries . . ." [In Mease' Picture of Philadelphia, 1811. pp. 330-31]*

Two views from the computer model reveal the relationship between the stairs in the west "projecting wing" and the remainder to the west portion of the building.