Here are five steps to understanding the EU Customs Union.
Number one, it’s the biggest customs union in the world.
A customs union is a trade agreement between countries.
That means no customs duties, no quotas or declarations as goods move around the member states.
They collectively agree on trade deals with countries outside the union.
Now, the customs union is different to the single market,
which is a broader agreement to accept free movement of services, capital and people as well as goods.
Number two, the trade between member states in the EU Customs Union is as frictionless as possible.
For instance, car manufacturers can import parts quickly from countries within the EU without fees and paperwork.
Outside the EU Customs Union, businesses could face extra fees and border checks.
So when the UK leaves it, it could slow the arrival of parts to the UK and delay production.
Some say this could force firms to quit the UK, causing job losses.
It could also make the price of goods more expensive if companies import goods have to pay more duty.
Others say new trade deals with new partner around the world would mean more service jobs in textiles, banking and design, and cheaper prices of goods.
Number three, The UK won’t be the first European country that is outside the EU Customs Union.
The 28 EU member states - including the UK - are in as well as Monaco, the Channel Islands and the Isle of Man.
Norway, Iceland and Liechtenstein are inside the single market and Switzerland is almost fully in it.
Gibraltar and the British Overseas Territories are outside both.
Then there is Turkey, Andorra and San Marino.
They’re outside the EU Customs Union.
But they have seperate customs agreements with the EU covering certain goods.
Number four, the UK wants to be outside both the single market and the EU Customs Union.
It wants to avoid a UK-EU customs border so as not to have trade hindered by customs fees and paperwork.
But it also wants to have the freedom to do trade deals with the rest of the world.
Michel Barnier, the EU’s Chef Brexit negotiator, says:"No".
The UK can’t have frictionless trade if it leaves both the customs union and the single market.
Number five, leaving the EU Customs Union could affect the UK’s land border with Ireland.
So when the UK leaves the EU Customs Union and Ireland remains in it, there will need to be a new border agreement.
Both sides want to avoid a hard border for goods and movement.
But so far, there’s been no agreement on a solution