Use of the title "Heir of Norway" ("Arving til Norge") established in the 17th century. Firstly, several junior agnatic members of the House of Oldenburg (the Duke of Holstein-Gottorp among the first ones), themselves usually titular Dukes in Schleswig-Holstein, assumed the title to constant use, as one of their main titles. There are plenty of examples from official dispatches and notices of the 17th, 18th and 19th century of several princely persons titled "Duke of Holstein, Heir of Norway". This is why it has been used as part of their titles by the emperors of Russia until 1917 since their agnatic line traces back to Peter III of Russia, first Russian ruler of the House of Oldenburg.