High Quality Content by WIKIPEDIA articles! 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 the feudal customs, and often meant a veritably independent principality. The title was gradually replaced by a nominal royal title Grand Duke of Finland by the early 17th century and has not been in usage since.