All frames forward of the forward perpendicular are identified by a capital letter, starting with A. These frames are identified by starting with the first frame forward of the forward perpendicular and working forward. The frames aft of the aft perpendicular are identified with double capital letters, starting with AA. Starting with the first frame aft of the aft perpendicular and working aft identifies these frames. The frames between the forward perpendicular and the aft perpendicular are identified by numbers. The forward perpendicular is identified by the number 0 (zero). Each frame aft of the forward perpendicular will carry the next higher consecutive number. The last numbered frame is the aft perpendicular. If the forward boundary of a compartment is located between frames, the frame number farthest forward within the compartment is used. Compartments located on the ship’s centerline carry the number 0. Compartments completely to starboard are given odd numbers, and those to port are given even numbers. Where two or more compartments have the same deck and frame number, they have consecutively higher odd or even numbers, as applicable, numbering from the centerline outboard. In this instance, the first compartment to starboard is 1, the second is 3, and so on. To port of the centerline they are numbered 2, 4, and so forth. When the centerline passes through more than one compartment, each of which has the same frame number, the compartment having the forward bulkhead through which the centerline passes carries the number 0; the others are numbered 01, 02, 03, as applicable.
The last part of the compartment number is the letter that identifies the primary usage of the compartment. On dry- and liquid-cargo ships, a double letter is used to designate cargo spaces. The double letter will differentiate them from spaces containing the same commodity for use by the ship. Fuel oil and JP-5 jet fuel are two examples.