A shelter deck is the deck forming the top of an enclosed area above the main deck. The enclosed area will have either openings through the shelter deck itself, sides, or ends which it encloses. These openings generally have weather-tight or spray-tight closures, but are not considered waterproof for the purpose of classification or flag state. Shelter decks are often used as a way of excluding enclosed volume from tonnage calculations, a form of "rule beating". Passenger ships, particularly on ocean liners, often have areas outboard of the deckhouse with lounge chairs and spray-tight windows which can be considered as a shelter deck.