Ships over 24m long doing international business have to respect the International Convention on Load Lines. Such vessels have a deck named the Freeboard Deck. It goes from end to end in the ship and from one side to the other. Decks below the weather deck can be the Freeboard Deck, as in most ferries. The distance from the sea surface to the Freeboard Deck is named the freeboard. All these ships have a minimum freeboard. At no time may they be so deep in the water that the freeboard is smaller than the minimum freeboard. For that reason, the Freeboard Deck is an important reference point in the ship. Some shipyards and shipping companies name the Freeboard Deck as Deck 1 or 1st Deck. The decks below are 2nd Deck or Deck 2, 3rd Deck or Deck 3 etc. The decks over the Freeboard Deck have letters. The first level over the Freeboard Deck is Deck A. Over that is Deck B, Deck C etc. This system of deck names says where the deck is in the ship. This is important because the Load Line Convention has special rules about doors, windows, hatch openings and ventilation openings on Deck A and Deck B.