Istanbul, historically known as Constantinople and Byzantium, is the most populous city in Turkey and the country's economic, cultural, and historic center. Istanbul is a transcontinental city in Eurasia, straddling the Bosphorus strait (which separates Europe and Asia) between the Sea of Marmara and the Black Sea. Its commercial and historical center lies on the European side and about a third of its population lives on the Asian side. The city is the administrative center of the Istanbul Metropolitan Municipality. Istanbul is one of the world's most populous cities and ranks as the world's 7th-largest city proper and the largest European city.