This paper attempts to demonstrate the rules governing the packing of rectangles into a general rectangular perimeter. The properties of a diagram expressing the relationships between the exterior lines and the interior lines forming such packings are shown. A classification of different kinds of arrangements emerges from these relationships and from the number, type, and disposition of the junctions produced when those lines meet together. The rules described can be of architectural interest since they allow some understanding of the combinatorial nature of a certain class of orthogonal building plans.