Above all, don’t forget this point. The whole thing could fall apart over something we haven’t even thought of yet.
It could also unravel in areas where things are very obviously still hanging open at the seams – most notable being on Northern Ireland.
The EU’s chief Brexit negotiator, Michel Barnier, indicated the UK had finally committed to a “backstop solution” to the Northern Ireland problem, which guarantees regulatory alignment and therefore frictionless trade across the border with the Republic of Ireland if no other agreement is reached.
The fear of the backstop solution is it looks rather like the EU’s preferred end state; essentially keeping Northern Ireland in the EU’s customs union and single market after Brexit.
With the UK signed up to this, some fear it could demotivate the EU from agreeing on anything else.
Theresa May initially said that no British prime minister could agree to this deal, because it would likely see new border checks emerge between Northern Ireland and the rest of the UK.
However, don’t be fooled into thinking this “if all else fails” solution has been fully agreed on.
The draft agreement document is coloured green where there is consensus and white where there is not.
Text relating to this thorny issue, stating Northern Ireland “shall be considered part of the customs territory of the Union”, remains stubbornly white.
The UK Brexit department insist they do not agree with the language of this and there is clearly still work to be done.
The success of the day was that the framework for the transition period was laid down, giving business certainty the UK will remain in the EU’s single market and customs union for an extended and defined period.
This avoids the so called “cliff-edge” for firms that are trying to plan financial decisions that stretch beyond March 2019.
They now have until the end of December 2020.
The cliff has been pushed back, giving politicians time to install a slide or prepare some big mattresses.
In order to reach this agreement there have been concessions on both sides; some red lines appear to have been lost in transition.
The Prime Minister always insisted “free movement will end in March 2019”.
Only a few days ago, the Government line was that EU nationals would be arriving with “different expectations” during the transition period.
This is no longer the case.
EU citizens arriving in the UK, up until December 2020, will continue to maintain their rights to remain in the UK.
The pay-off for this is that the EU has accepted something they said would never happen, which is that the UK will be able to negotiate and ratify international trade deals while remaining in the customs union.
They hope one of these will be with the EU itself.
However, these deals can only be implemented once we have left the customs union, at the end of the transition period.
This goes some way to avoiding what leading Conservative Brexiteer Jacob Rees-Mogg described as the UK becoming a “joke nation” during a transition period.
A controversial area of Monday’s agreement is fishing.
Here, the UK has agreed to remain signed up to the EU’s Common Fisheries Policy (CFP) during the 20-month limbo period.
However, during this time Britain will only be “consulted” on CFP decisions.
The Scottish Fishermen’s Federation said in a statement: “This falls far short of an acceptable deal.
“We will leave the EU and leave the CFP, but hand back sovereignty over our seas a few seconds later.
“Our fishing communities’ fortunes will still be subject to the whim and largesse of the EU for another two years.”
In one concession, Brexit Secretary David Davis has ensured the UK’s fishing share cannot be changed during this time.
Overall, there was a generally upbeat mood in both camps.
There has been movement – perhaps most notably on citizens’ rights – but big questions have yet to be answered.
Many haven’t even been discussed. But they can be now.
The most significant achievement is the UK has finally jumped all the EU’s hurdles before being allowed to discuss the future relationship with the bloc.
That can now begin.