Duke of Finland (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, which was revived again briefly from 1802 to 1805 (then as Grand Prince of Finland) and was also used by Russia's monarchs until 1917.