As the UK tries to decide whether it's in or out, we take a quick look at the European Customs Union, and ask, why have one?
ADVERTISEMENTHow will the UK conduct its trade after Brexit?
Its Government has said it wants to leave the European customs union, but faces mounting pressure both from the opposition and from within its own party to stay inside.
A Customs union is a type of Free Trade Agreement, which sees the removal of tariffs between members, and the imposition of a common, centrally-negotiated tariff against non-member countries.
States don't have to belong to the EU to opt into the customs union. Andorra, San Marino and Turkey have all chosen this path, and Gurnsey, Jersey, the Isle of Man and Monaco have signed treaties that place them inside the union, too.
British sceptics have cast doubt over the benefits enjoyed by Turkey in this arrangement, claiming they have to accept EU trading rules without having any say in them.
The advantages include increased trade between members.
For less wealthy countries, such as Bulgaria, this also puts collective European muscle on their side of the negotiating table.
For some industries, such as car manufacturing, it removes the complications of components which sometimes cross borders multiple times.
However, countries within the union aren't free to negotiate their own deals with non-member countries, such as China, a constraint that frustrates UK Brexiteers, and which was one of the main reasons for the country voting to leave the EU in the first place.