The representation within a computer of such complex entities as ships is described. The representation is so constructed that it may be built up and changed as the design of the entity progresses. An entity may consist of any hollow polyhedron which is divided in two practical ways into compartments or spaces. The faces of the polyhedron and the divisions may be plane or curved and are deemed to have no thickness. Although quite general in its application this representation is well suited to the description in a computer of the hull and superstructure of a ship and its decks and bulkheads during the early stages of design. Algorithms are described for dividing any space within the ship into two spaces, moving or deleting any division, and defining any space for the calculation of areas and volumes. Application of these in a computer-aided ship design system is described and some examples given.