A duke can either be a monarch ruling over a duchy or a member of royalty or nobility, and is historically of highest rank below the monarch. A duchy is the territory or geopolitical entity ruled by a duke, whereas his title or area is often called a dukedom. Dukes are the closest and highest ranking peers of the king and they were usually relatives of a sovereign family. The name duke originates from the Latin word Dux, which translates to 'leader'. It was a title used by the Roman Republic to refer to a military commander without official rank.