The Montreux Convention lets merchant ships pass the Turkish Straits freely but bars aircraft carriers

Background

The Montreux Convention has been implemented since 1936 and governs passage through the Turkish Straits.
Merchant vessels have freedom of passage through the Straits.
Aircraft carriers cannot pass through the Turkish Straits, whether or not they belong to a Black Sea littoral state.
Only submarines of littoral states may pass, and only to reach their Black Sea base for the first time after construction or purchase, or for repairs in dockyards outside the Black Sea.
Foreign naval forces in transit are limited to 9 vessels with an aggregate tonnage of 15,000 tons.
Non-littoral states may keep warships of up to 45,000 tons in total in the Black Sea, with no more than 30,000 tons from any one state, and such warships may stay no more than 21 days.
Warship passage must be notified to Turkiye through diplomatic channels 8 days in advance for littoral states and 15 days in advance for non-littoral states.
The Turkish Straits Vessel Traffic Services Centre manages merchant vessel passages under the 1998 Maritime Traffic Regulations.