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The European Union's (EU's) single market is an agreement designed to make it as easy for people, goods, services and money to move around between countries that are a part of it.

According to the EU's official website, the idea is that these four things "can move around the EU as freely as within a single country".

"EU citizens can study, live, shop, work and retire in any EU country - and enjoy products from all over Europe," says the site.

It is sometimes referred to as the internal market.

Which countries are in the single market?

All 28 countries in the European Union (EU) are part of the single market.

But some other countries also have arrangement with the EU, which means they benefit from it.

Norway, Iceland and Liechtenstein are part of an agreement called the European Economic Area (EEA), alongside the 28 EU members. This allows them to be part of the single market, while not being part of the EU.

Just to complicate things further, you can be in the EU's single market, but not in the EU and not in the customs union.

Switzerland has another single market agreement with the EU. Their agreement has lots of similar elements to other countries' agreements - for example, Swiss people can live and work elsewhere in the EU - but the agreement is specific between the EU and Switzerland.

Some EU countries have slightly different rules within the single market though.

For example, Bulgaria, Croatia, Cyprus, Ireland, Romania and the UK did not sign up to something called the Schengen Treaty, which made border controls more relaxed for other single market countries.

The UK also did not adopt the euro as its money, unlike many other EU countries.

What's the single market got to do with Brexit?

There are lots of conversations going on at the moment about what the UK's relationship will be like with the EU when it leaves the group.

This includes how the UK will be involved - if at all - with arrangements like the single market, as the UK will no longer automatically be a part of these after Brexit.

One of the key single market issues being discussed in relation to Brexit is the freedom of the movement of people (immigration and emigration).

The single market's policy of free immigration between member countries policy was one of the big issues discussed during the referendum campaign, as some people want the UK to have more control of who can come to live and work here.

The single market and the customs union

The single market and the customs union are not the same thing, but you might have heard them being talked about together.

The customs union is to do with the money charged for goods and services at borders.

The flow of goods and services is just one part of the single market.