Assuming the deal is voted down, the margin of defeat is important.
Should May lose by around 30, 40 or perhaps even 50 votes then it's expected she could stand up in Parliament and say she will return to Brussels and attempt to tweak the deal.
If slight concessions were agreed with the EU, that could be enough to persuade some U.K. lawmakers to side with May in a second parliamentary vote.
One caveat to that scenario is that the rest of the EU have shown little appetite to reopen talks. European Commission President Jean-Claude Juncker has warned the U.K. that May's Brexit deal "is the only deal possible."
Should May lose heavily, then chaos is once again likely to descend upon British politics.
The range of outcomes runs from a leadership challenge, a May resignation, a general election in the U.K., a second referendum on EU membership, a temporary stop on Britain's withdrawal, or some sort of combination of all the above.