The European Union is already operating at several different speeds, even before Britain's vote to leave raised new questions about whether the bloc should be more integrated or give more power back to its member countries.

That reflects differing levels of ambition and national sensitivities in what started as a club of six western European nations in 1951 and expanded gradually into a union with 15 members by 1995. A large group of formerly communist eastern countries followed a decade later, with the most recent addition — Croatia — bringing membership to 28 nations in 2013.

Here's a look at some of the different groups within today's EU:

THE EURO CLUB

Nineteen of the EU's members use the euro currency, which was introduced for transactions in 1999 and as cash in 2002.

Britain and Denmark won the right to opt out of the euro when it was conceived. All other EU countries are, at least on paper, obliged to join at some stage. However, Sweden rejected the euro in a 2003 referendum, and while several of the EU's newer eastern members have joined, others show no sign of doing so. Governments in Poland, Hungary and the Czech Republic are paying lip service at most to the requirement to join.

The euro currency zone is still working to put behind it the debt and economic problems that threatened it with collapse and forced it to bail out Ireland, Greece, Portugal, Spain and Cyprus.

The members of the eurozone are: Austria, Belgium, Cyprus, Estonia, Finland, France, Germany, Greece, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Portugal, Slovakia, Slovenia and Spain.

___

OPEN BORDERS

The overwhelming majority of EU nations — as well as four countries that aren't actually members — are part of the continent's borderless travel area. It's called the Schengen area after the village in Luxembourg where the treaty on travel was signed three decades ago.

Anyone entering the Schengen area must undergo an ID check against a customs and criminal database, but once inside there are no border checks. However, countries can re-establish them temporarily in exceptional circumstances. That typically happens for security reasons, such as during major summits, and has also been done during the past year's migrant influx. The no-checks system also permits the smooth flow of goods, services and business expertise.

Britain and Ireland opted out of Schengen while four of the EU's newest members want to join but haven't yet got the go-ahead.

The Schengen area's members are: Austria, Belgium, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Poland, Portugal, Slovakia, Slovenia, Spain and Sweden — plus non-EU countries Iceland, Liechtenstein, Norway and Switzerland.

___

TAXING TRANSACTIONS

A group of 10 EU countries including eurozone heavyweights France, Germany, Italy and Spain decided after the financial crisis to introduce a financial transaction tax — which they believe would curb speculation and claw back revenues after governments had to bail out banks. However, they have yet to reach an agreement on how to go about introducing the tax, which Britain, Sweden and others rejected from the start.

Another product of the economic crises of recent years is the Fiscal Compact, a German-championed agreement that introduced stricter requirements for countries' budgetary discipline. Everyone in the EU except Britain and the Czech Republic signed up.

___

OPT-OUTS

Various countries have wrung the right to opt out of EU legislation on individual issues. Britain has been a leader in this; along with Ireland and Denmark, it negotiated an opt-out from a set of common rules on immigration. Britain and Poland also obtained opt-outs guaranteeing that the EU's Charter of Fundamental Rights — a list of liberties that included collective labor bargaining and fair working conditions — would not create greater social or economic rights than those already provided for by local law.