Since then, most airlines have collected themselves into three different alliances: OneWorld, Star Alliance, and SkyTeam.

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Before Open Skies, the SkyTeam alliance offered no transatlantic routes to and from Heathrow.

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SkyTeam carriers will be located for the first time in Terminal 4.

CNN: Open Skies explained

Membership in a major alliance, including Star Alliance, SkyTeam and oneworld, can also be considered a reassurance to nervous travelers, Voss said.

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Delta and Aeromexico are both part of the SkyTeam alliance and have about 500 code shares that would rise to 700 under the enhanced partnership.

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Korean Air's Morning Calm members need to fly 50, 000 miles to become SkyTeam Elite members, while their Delta brethren earn the same privileges with just 30, 000.

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But it seems curiously reluctant to make progress on its investigation of the Star and SkyTeam cartels, perhaps for fear of opening a row with the United States.

ECONOMIST: Airline alliances

Most carriers of any size have joined one of the three big airline alliances oneworld, SkyTeam and Star but Virgin has stayed apart, and now the alliances may no longer feel inclined to woo it.

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The biggest challenge to the incumbent airlines operating out of Heathrow will come from carriers in the SkyTeam alliance, that includes Air France and KLM, Delta Air Lines, Northwest Airlines News, and Continental Airlines.

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The European Commission, the EU's regulator, has been a bit tougher on the airlines than have the American authorities: before approving oneworld's transatlantic venture, it made its members surrender slots at Heathrow, some of which were snapped up by Delta, a SkyTeam member.

ECONOMIST: Airline alliances

If it were to lose United, its biggest member, there would be such a scramble to replace the American partner that the two other big alliances (SkyTeam and oneworld) could be torn apart as Star woos Delta Air Lines and American Airlines, their American anchors.

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Speaking on BBC Radio this morning, IAG's boss, Willie Walsh, made the point that this would be a far lower share of slots than Lufthansa and its Star Alliance partners have at Frankfurt, or that Air France and its fellow SkyTeam airlines have at Paris Charles de Gaulle.

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