1. BY THE BOOK
Prime Minister David Cameron, who said he will resign after losing his gamble to end British ambivalence about staying in, agrees with the EU establishment that the only legal way to leave is to use Article 50 of the EU's Lisbon Treaty to negotiate a withdrawal.
He wants to leave triggering the process to his successor, who may not be chosen by the Conservative party until October. EU leaders want him to do it now, or at least as soon as possible, but they lack the legal power to force him.
In the most amicable divorce scenario, Britain would trigger Article 50, possibly (though unlikely now) as early as Tuesday when Cameron meets the other 27 EU leaders at a Brussels summit, or via a formal letter later from Cameron or his successor.
That sets a two-year time limit on negotiating an amicable withdrawal. Ideally, it would divide up assets and liabilities in the shared EU budget and other priority business, such as perhaps the status of British and other EU citizens who find themselves living on the wrong side of a hard new UK-EU border.
In an even more ideal world, it would set out a new, close economic relationship between Britain and the EU, possibly in a separate, parallel treaty taking effect from the exit date. The withdrawal treaty can be enacted by just 20 of the 27 other states representing 65 percent of the remaining population. A full new relationship would probably need unanimous support.
Two years is very tight but the negotiations can be extended if all 28 countries agree. If there is no deal, then Britain is simply out of the EU two years after Article 50 was triggered -- an outcome written in to the treaty to limit uncertainties.
REALLY? TOO GOOD TO BE TRUE. THE EU IS NEVER THIS EASY
2. SORRY, WE DIDN'T MEAN IT
Britain is in political meltdown, with both main parties in civil war and pro-EU Scotland threatening to either block Brexit legally (unclear how) or break away. The referendum result is not constitutionally binding and government and parliament, maybe after a new election, could just ignore it. If so, the EU would carry on as before but a special membership deal it gave Cameron in February has been killed by the referendum result.
REALLY? PUSHES DEMOCRATIC CREDIBILITY BEYOND BREAKING POINT
3. WE MEAN IT, BUT NOT YET
Brexit campaigners have long been suspicious of the two-year limit in Article 50 and some have explicitly said it should only formally be triggered AFTER they have agreed a comprehensive free trade deal that relieves Britain of EU rules such as open EU immigration. Five years or more is the norm globally for such big trade deals. Britain would be a full EU member until then.
That is a nightmare scenario for EU leaders, plunging the bloc into open-ended negotiations with its second biggest power that would inspire eurosceptics across the bloc to emulate it and distract governments from other pressing European issues.
They rule out opening any negotiation until Britain binds itself to the timetable set out in Article 50. And they insist Britain cannot have its cake on market access and still eat it by ending EU budget payments and free movement of workers.
In theory, there could be an endless standoff, with Britain the sulky teenager at the table, poisoning the atmosphere next year as France and Germany run elections and the EU starts confrontational talks on a new 7-year budget. Something would have to give and some compromise would start to be worked out.
REALLY? COMPROMISE IS THE EU WAY; DON'T RULE THIS OUT
4. WE MEAN IT -- OR MAYBE WE DIDN'T
Article 50 suggests a one-way exit, rather than a revolving door. EU officials insist that once triggered, a state cannot back out and stay. Lawyers are divided, however. Some British experts believe the leave notice could simply be withdrawn. In Brussels, others say that could happen but only if agreed by all. A future British government might conclude that the best way to end divorce proceedings is just to agree to stay married.
REALLY? SEE SCENARIO 2, BUT WITH ADDED FADING OF MEMORIES
5. CAN WE JUST TWEAK THIS QUICKLY?
Some Brexit campaigners have suggested that the Leave vote simply serve as leverage to renegotiate better, semi-detached terms for Britain inside the EU which could be put to another referendum. EU leaders have ruled that out on the same grounds as above that "cherry picking" will spread and wreck the Union. Cameron's deal, to protect the City of London from the euro zone and curb EU immigration, has been killed by a clause that linked it to last week's referendum result. So any talks would start from a lower base and EU leaders would have to eat their words. But some kind of "associate membership" or "special partnership" has been around as an idea in Europe for a time.
REALLY? SEE 4, BUT NEVER SAY NEVER IN EURO-COMPROMISE LAND
6. LET'S JUST SLIP INTO SOMETHING MORE COMFORTABLE
Britain could try to join the European Economic Area or European Free Trade Association, joining the likes of Norway, Switzerland or Iceland in close partnerships with the EU. That could fly with the EU but British leaders would have to persuade Brexit voters to agree to the EU budget contributions and migrants that are accepted by some of those countries. It also would lack the kind of EU market access for services trade which is so important to Britain's big financial sector. A more tailor-made deal would bring things back to earlier scenarios.
REALLY? DOESN'T SEEM TO BE WHAT BRITONS WANT, AT LEAST NOW
7. WHY DON'T WE START AGAIN?
One extreme view is that the fallout from Brexit in the EU might be so cataclysmic that Europeans would go back to the drawing board and effectively create a new kind of Union that could include Britain. Marshalling disparate national ambitions into a new structure would be a colossal task, not least in the wake of the bitterness that the current crisis has engendered.
REALLY? A DEFINITE LONG SHOT, NOT ONE FOR THE NEAR FUTURE
8. SECOND TIME LUCKY?
Some people who voted to Leave have said that if it doesn't work outside, Britain could always join the EU again. That is true, though it would get no favors. It would face a years-long accession process and require unanimous acceptance by existing members and have to accept a host of conditions that Britain has opted out of during its past 43 years -- notably adopting the euro and a virtually 50-percent rebate on EU membership fees.
REALLY? ONE FOR THE (VERY LONG-LIVED) BIRDS
9. ROOM FOR A LITTLE ONE?
Some Scots hope to avoid Brexit by breaking from England. An idea that an independent Scotland could somehow simply sit in the vacant UK chair in Brussels is dismissed by EU officials.
At the time of the 2014 independence referendum, the EU said Scotland could apply for membership but would go to the "back of the queue", behind the likes of Serbia, and that its hope of keeping the pound rather than the euro was a non-starter. There is some sympathy for Scots losing their place in the EU but also deep suspicion of secession, especially in Spain, Belgium and Italy, which have their own separatist problems. And a veto.
REALLY? NOT SOON, BUT SCOTLAND COULD JOIN THE EU ONE DAY
10. SLAMMING THE DOOR
A new British government simply walks out. It could launch Article 50 and leave legally in 2019 without any negotiation. It could also ignore the two-year notice period and tear up its treaty obligations and quit right away, though that would undermine its credibility as a party to international law. There is, however, nothing the EU can do to prevent that.
It could retaliate on trade or against Britons living in the EU, however much that would create a painful tit-for-tat that would badly hurt Europe's economy and citizens. Nonetheless, EU leaders fear that letting Brexit Britain walk all over them will only inspire other European nationalists to destroy the Union.
REALLY? THREAT OF MUTUALLY ASSURED DESTRUCTION THEN DETENTE?