Complex sentence: The Duchy of Pomerania (12th century-1637) was a duchy in Pomerania on the southern coast of the Baltic Sea, ruled by dukes of the House of Pomerania ("Griffins").
Split into simpler sentences: The Duchy of Pomerania was a duchy in Pomerania on the southern borders of the Baltic Sea. It existed from the 12th century till mid 17th century and was ruled by dukes of the House of Pomerania ("Griffins").