There are two floors beneath the bottom floor of the Pentagon. The two underground floors are labeled "M" for "Mezzanine" and "B" for "Basement."
The Pentagon has five floors above ground. Most people think of the Pentagon's second floor as its main floor, due to the Concourse being located on this floor.
The concentric rings around the Pentagon hold offices constructed in a clockwise formation. The rings are labeled beginning at their innermost point, which is ring A, out to the outermost point, which is ring E. Notably, the Pentagon houses offices for 25,000 military and civilian personnel.