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On which two continents does Istanbul sit?
The city straddles the Bosphorus strait.
Istanbul is the best-known city on two continents because the Bosphorus, a strait about 30 kilometers (roughly 19 miles) long, separates its European and Asian districts. By convention the strait is part of the boundary between Europe and Asia, linking the Black Sea in the north to the Sea of Marmara in the south. The European side holds most of the historic core, including the old peninsula of Sultanahmet, while roughly a third of residents are usually reported to live on the Asian side.
The strait is crossed daily by ferries and by several road bridges and rail tunnels, including the Bosphorus Bridge and the Fatih Sultan Mehmet Bridge, as well as the undersea Marmaray rail tunnel, which opened in 2013. Commuters in Istanbul therefore literally change continents on the way to work.
The 'Europe-Asia' boundary itself is a human convention rather than a geological fact, since both continents are part of the single landmass Eurasia. Other conventional boundaries run along the Ural Mountains, the Caucasus and the Turkish Straits. Istanbul is not unique: a few other cities, such as Orenburg on the Ural River and Atyrau in Kazakhstan, are also commonly described as spanning the line, though Istanbul is by far the largest.