Desperate to halt the flow of migrants fleeing conflict or poverty, some European countries have begun erecting fences and tightening border controls. But the crackdown, as people leave Greece and Italy to move deeper into the European Union, has raised questions about whether Europe's passport-free area is viable.

The no-checks zone, known as the Schengen area, does not only allow individuals to travel freely across borders. It also permits the smooth flow of goods, services and business expertise — in a vital boost to the European economy.

Here are a few things about it to bear in mind:

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EUROPEAN, NOT EU

Schengen is named for the Luxembourg village where the border treaty was signed 30 years ago. It has 26 members; 22 of the 28 EU nations plus Switzerland, Norway, Lichtenstein and Iceland. EU states Cyprus, Croatia, Romania and Bulgaria, Britain and Ireland are not in, although some want to join. Border controls between members have been coming down for about 20 years. Schengen is likely to grow and its rules, which were last modified in 2013, will evolve as well.

HOW IT WORKS

Anyone entering the Schengen area must undergo an ID check against a customs and criminal database. That can be on borders around Europe's perimeter, or at a Schengen member's airport, say for Americans flying in to Paris. But once inside, no ID checks with any database are allowed. The EU's executive Commission supervises application of the Schengen agreement and can intervene if countries start to take action that might substitute for border controls.

INTRA-EUROPEAN FLIGHTS

Passport checks at European airports are done by air carriers for commercial reasons, not by customs and borders officers. They want to make sure the traveler is the same person named on the ticket. These kinds of checks are allowed as long as the ID is not swiped into a database reader. In the wake of the high-speed international train attack late last month, the EU is considering printing people's names on their tickets.

SECURITY CHECKS

After the deaths of 71 people who suffocated in a truck last week, Austrian police stepped up border security, causing huge traffic jams. Security controls are not linked to Schengen, and national police are free to conduct whatever checks they think are necessary. They can also check people's ID, as long as they do not check documents against a computer database, except when they suspect the person may be involved in a crime.

BORDER CLOSURES

Schengen countries can in exceptional circumstances and for a limited time close their borders, mostly for security reasons. Germany did this in June for the G7 summit. Major terror attacks and potentially a massive migrant influx could be invoked as a reason for doing so. Nations must inform the Commission of their intentions and reasons.

THE "DUBLIN" REGULATION

The Dublin rules were first signed in the Irish capital in 1990 and are having a huge impact on the way migrants are shared around. Under Dublin, a person must apply for asylum in the Schengen country they first arrive in. In practical terms, this mainly means Greece and Italy at the moment. That's why migrants who want to live elsewhere refuse to be processed in Greece and Italy, and why those country's reception centers are overwhelmed. Countries can decide not to apply this rule and allow people to apply for asylum on their territory instead, as Germany has just done for Syrian refugees.