“No sovereign state would allow part of its territory to belong to the customs territory of another state”. That (in the context of #Brexit and NI) sounds rather reasonable and might be cause of a red line. But is it true? (Thread)
I present to you Büsingen. Büsingen has a bit of an unfortunate location for being part of Germany... 
Accordingly, under Art. 4(1) of the Union Customs Code Büsingen is not part of the EU Customs territory. In fact, under a 1964 treaty (attached) between Germany and Switzerland, the German territory of Büsingen is part of the Swiss customs territory. admin.ch/opc/de/classif…
An interesting comparator is Campione d’Italia. An Italian exclave in Switzerland, similarly not included in the EU customs territory under Art. 4(1) of the Union Customs Code.
Italy and Switzerland have not concluded a treaty including Campione in the Swiss customs territory. However, it is de facto included in that territory.
The exclusion of, e.g., Büsingen from the EU customs territory means that if you travel from the German city of Büsingen to Germany, you will be subject to extra-EU treatment. Any doubts? Here’s the website of the German Zoll stating that explicitly zoll.de/DE/Privatperso…
The reason for these exceptions are geographical and historical. Going through Art. 4(1) of the UCC you’ll find member state territories excluded from the EU customs terriory, 4(2) lists non-EU member states included.
4(2) lists Monaco and - yes, another Brexit favourite - the UK Sovereign Base Areas in Cyprus.
So territory of one country is, at times, included in the customs territory of another for geographical and historical reasons. Of course that doesn’t mean it must be.
