The Bosporus or Bosphorus (Turkish Bogazici) is a strait that separates the European part of Turkey from the Asian part, and connects the Sea of Marmara with the Black Sea. The ancient Greeks referred to this strait as the Thracian Bosporus, as they called the Strait of Kerch the Cimmerian Bosporus. Increasing the chances of confusion, they also called a land area near these two straits by the same name: the Thracian Chersonesus, which is known today as Gallipoli, and the Cimmerian Chersonesus, known today as the Crimea.
Due to the importance of the strait for the defense of Istanbul, the Ottoman sultans constructed a castle on each side of it (Anadoluhisari and Rumelihisari). Its strategic importance remains high: several international treaties have governed vessels using the waters. The current one is the Montreux Convention Regarding the Regime of the Turkish Straits, signed in 1936.