A starship is a large, space-faring spacecraft, usually with multiple decks. A starship usually has sufficient size to carry its own boats, such as lifeboats, dinghies, or runabouts. A rule of thumb saying (though it doesn't always apply) goes: "a boat can fit on a starship, but a starship can't fit on a boat". Often local law and regulation will define the exact size (or the number of masts) which a boat requires to become a ship. (Note that one refers to submarines as "boats", because early submarines were small enough to be carried aboard a ship in transit to distant waters.)