Duke of Finland (in Finnish Suomen herttua; Swedish hertig av Finland) was an occasional medieval title granted as a tertiogeniture to the relatives of the King of Sweden between the 13th and 16th centuries. It included a duchy along with feudal customs, and often represented a veritably independent principality. Grand Duke of Finland was a nominal royal title used by Swedish monarchs from the 1580s until 1720, was revived again briefly 1802-1805 (then as Great Prince of Finland) and was also used by Russia's monarchs until 1917.