Istanbul, formerly known as Byzantium and Constantinople, 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 Bosporus strait 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 in suburbs on the Asian side of the Bosporus. With a total population of around 15 million residents in its metropolitan area, Istanbul is one of the world's most populous cities, ranking as the world's fifth-largest city proper and the largest European city. The city is the administrative center of the Istanbul Metropolitan Municipality.