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 fifteen million residents in its metropolitan area, Istanbul is one of the world's largest cities by population, ranking as the world's fifteenth-largest city and the largest city in Europe. The city is the administrative centre of the Istanbul Metropolitan Municipality.