Briefly explained: Montreux Convention

Turkey has the straits Bosporus and the Dardanelles closed to Russian ships.

On 24.02.22, Ukraine called on Turkey to take this measure. Various agencies then reported that Turkey had authorised passage into the Black Sea.closed. On 26.02.22, Reuters reported that this was a rumour and in fact no official announcement had been made at that time. The confusion has now ended. On 28.02.22, Ankara announced that NATO member Turkey was preventing Russian warships from calling at Ukraine via the Black Sea. President Erdogan had previously condemned the attack on Ukraine.

Turkish Foreign Minister Cavusoglu referred to the 1936 Montreux Treaty, but what does it say?

The Treaty of Montreux of 1936 gives Turkey full sovereignty over the Dardanelles, the Sea of Marmara and the Bosporus. The treaty consists of 29 articles regulating civil and military shipping traffic. This also includes the regulation of fees, the definition of warships and the total tonnage. The straits are an international shipping lane, which means that merchant ships have free passage in peacetime. The Turkish authorities can levy charges but cannot prohibit passage. In times of war, all merchant ships from countries that are not at war with Turkey may pass through.

Special regulations apply to warships: In times of peace, the Turkish authorities must be notified in advance through diplomatic channels of the passage of a warship. Warships from countries that do not border the Black Sea may not stay in the Black Sea for more than 21 days. The tonnage of warships from non-river states may not exceed 15,000 tonnes in total, with individual exceptions. If Turkey is at war, sovereignty lies with Turkey, but if - as in the current case - there is a war in which Turkey is not involved, it may prohibit the passage of warships from belligerent nations. The exception is the return to the home port.

Source: Reuters, Wikidark, archive.org/maritimelaw