Northern Ireland will have a
different relationship with the EU than the rest of the UK from 1 January, because it will remain in the EU single market for goods.
That was part of the plan contained in the
Brexit withdrawal agreement to keep the Irish land border between Northern Ireland (in the UK) and the Republic of Ireland (in the EU) as open as it is now.
Earlier this month EU and UK negotiators reached a separate agreement on how this will work in practice.
Goods manufactured in Northern Ireland will continue to have seamless access to the EU, but as a result new bureaucracy will emerge within the UK between Great Britain and Northern Ireland.
There will be document and physical checks on some food supplies, for example (although a three month grace period will begin on 1 January for many traders, before new measures are applied in full).
Pets being taken from Great Britain into Northern Ireland will also need an animal health certificate from a vet.
And because Northern Ireland is being treated differently, this agreement could have longer term constitutional significance for the UK.
Pro-independence politicians in Scotland will continue to ask why, if Northern Ireland can stay in parts of the single market, Scotland can't do the same.