A deck is a permanent covering over a compartment or a hull of a ship. On a boat or ship, the primary or upper deck is the horizontal structure which forms the 'roof' for the hull, which both strengthens the hull and serves as the primary working surface. Vessels often have more than one level both within the hull and in the superstructure above the primary deck which are similar to the floors of a multi-storey building, and which are also referred to as decks, as are specific compartments and decks built over specific areas of the superstructure. (Decks for some purposes have specific names; see below.)