‘The Montreux Convention on the Regime of the Straits,’ signed in 1936, addresses the rights granted to the recently christened Trukish state in regards to the Hellespont and the Bosphorus. This is a crucial region as it governs the Black Sea Powers access to the Mediterranean. In other words, it governs Russia’s access to the Mediterranean. And with Russia’s recent incursions into eastern Ukraine and their outright annexation of Crimea, Russia’s Black Sea interests have again become a Western concern.
The Convention limits the total tonnage of ships in transit through the Straits, so as to prevent entire fleets from entering the Marmara Sea; aircraft cannot take off from ships in transit; submarines must travel by day and above water; should a ship exceed the total tonnage allowed for all ships in transit, it can pass with no more than two destroyers as escort. Other regulations pertain to sanitation, etc. But these are the major laws regarding the transit of non-belligerent (to Turkey) vessels. The important thing to take away is that Turkey has relatively little control over the Straits.
The Convention prevents Turkey from blocking the Black Sea Powers or using it as leverage in economic negotiations. Naturally, this doesn’t sit well with the autocratic President of Turkey Recep Tayyip Erdogan. Erdogan seeks a Turkey more similar to the Ottoman Empire of old, but this is a tenuous fantasy when Turkey can’t even legislate the water next to its most populous city, Istanbul.
This will become an increasingly important Treaty as both Russia and Turkey try to expand their influence as they reach for their previous heights. Russia has already shown a penchant for (successful) irredentism and Turkey hasn’t exactly been a stalwart partisan in the fight against Islamism. Quite the opposite, in fact. The real concern here is an event like that of 24 November.
The fastest path toward escalation would be Turkey closing the Hellespont and the Bosphorus to Russia, which would be tantamount to declaring war as that is the only way they can close it to warships. Russia would certainly not suffer losing access to the Mediterranean via the Black Sea. This is the fastest route the Syrian conflict could take toward a more global, serious conflict.
The Convention itself is fairly short and worth reading: http://cil.nus.edu.sg/rp/il/pdf/1936%20Convention%20Regarding%20the%20Regime%20of%20the%20Straits-pdf.pdf