A ship prefix is a combination of letters, usually abbreviations, used in front of the name of a civilian or naval ship. Prefixes for civilian vessels may either identify the type of propulsion, such as "SS" for steamship, or purpose, such as "RV" for research vessel. Civilian prefixes are often used inconsistently, and frequently not at all. Sometimes a slash is used to separate the letters, as in "M/S". These tables list both current and historical prefixes known to have been used.