Here's a physical description of the Pentagon: it's a five sided, five story (plus two basements) building containing a large central courtyard and five concentric rings of offices.
Floors: numbered 1 through 5, except for the basements, which are labeled M (for Mezzanine) and B (for Basement). Note that most people would think of the second floor of the Pentagon as its "main" floor.
Concentric rings: labeled A (innermost) through E (outermost) except in the basement, where there are also ring segments labeled F and G. "E" Ring offices are the only ones with outside views and are generally occupied by senior officials.
Office numbers: office numbers go clockwise around each of the rings, and have two parts: a nearest-corridor number (1 to 10) followed by a bay number (00 to 99), so office numbers range from 100 to 1099. These corridors radiate out from the central courtyard, with corridor 1 beginning with the Concourse's south end. Each numbered radial corridor intersects with the corresponding numbered group of offices (for example, corridor 5 divides the 500 series office block).
Radial corridors: numbered 1 through 10, starting with the radial corridor off the south end of the concourse.
Thus office number 3E210 is on the third floor, in the E Ring, about two tenths of the way around the E Ring in the clockwise direction, starting from the middle of the concourse. The radial corridors go between the concentric rings (and thus radiate outward from the central courtyard) are found where two adjacent sides of the building come together. Oddly enough, the radial corridor numbering and the room numbering are connected. For instance, if you walk down corridor 7 to its intersection with the D Ring, you will find that offices on the D Ring to your left are numbered in the seven hundreds and that offices on the D Ring to your right are numbered in the seven hundreds.
The Concourse: The Concourse is on the second floor of the Pentagon. It sits where the E Ring would be on the southeast side of the building. There is a large Metrobus station about 200 yards outside the building next to the Concourse. The top of the escalators to the Pentagon Metrorail station is about 100 yards from the Concourse building entrance. This entrance is now the main entrance for visitors to the Pentagon. The Concourse itself is a mini-shopping mall located inside the Pentagon.