The Bosphorus separates Turkey’s European and Asian parts and connects the Black Sea with the Sea of Marmara.
The strait runs through Istanbul, making the city one of the world’s notable transcontinental urban centers. It is about 31 kilometers long, though its width varies considerably. The Bosphorus is also known as the Istanbul Strait in modern Turkish usage.
Its strategic position has shaped regional history for thousands of years. Control of the waterway mattered to Greek colonies, the Byzantine Empire, the Ottoman Empire, and modern Turkey. Ships traveling between the Black Sea and the Mediterranean must pass through the Bosphorus and then the Dardanelles.
The Bosphorus should not be confused with the Dardanelles. The Bosphorus links the Black Sea to the Sea of Marmara, while the Dardanelles links the Sea of Marmara to the Aegean Sea. Together, the two straits form part of the Turkish Straits system, a major international maritime route.