The Bosporus (Turkish Bogazici) is a strait which separates the European part of Turkey from the Asian part by uniting the Sea of Marmara with the Black Sea. There are two bridges across the strait: the Bogazici (Bosporus I) Bridge (1,074 metres in length and completed in 1973) and the Fatih Sultan Mehmed (Bosporus II) Bridge (1,090 metres in length, and completed in 1988). Due to the importance of the strait for the defense of Istanbul, Ottoman sultans constructed a castle on each side of the strait (Anadoluhisari and Rumelihisari).
The strategic importance of the straits is such that a number of international treaties have agreed to control vessels using the waters. The current treaty is 1936's Montreux Convention Regarding the Regime of the Turkish Straits.