Complex sentence: Adolf founded a cadet branch of the royal Danish House of Oldenburg called House of Holstein-Gottorp, a convenient usage for the technically more correct "Duke of Schleswig and Holstein at Gottorp".
Split into simpler sentences: Adolf founded a cadet branch of the royal Danish House of Oldenburg called House of Holstein-Gottorp. A convenient usage for the technically more correct "Duke of Schleswig and Holstein at Gottorp".