It's the key part of the Brexit lexicon but what is the Irish border backstop and why is it so important?
It is basically an insurance policy to guard against border posts returning between Ireland and Northern Ireland.
For many, it would be a reminder of a sectarian conflict that killed over 3,600 people.
During the three-decade conflict — known as the "Troubles" and which lasted from 1968 until 1998 — the border was militarised.
Amid the political and sectarian violence, the UK installed security infrastructure like watchtowers and army posts.
It all came down after the Good Friday Agreement was signed in 1998.
So how does Brexit pose a threat to this?
After Brexit, the frontier between Northern Ireland and the Republic of Ireland will become the only land border between the UK and the EU.
The Republic of Ireland will remain in the EU, while Northern Ireland, as part of the UK, will come out of the bloc.
The chances of a soft Brexit are looking extremely slim, so it is likely the UK and Northern Ireland will no longer be in the EU's single market or customs union.
If both sides have different regulations and customs rules, then enforcing those might require checks and controls.
Even though London and Brussels have said they don't want a return of border infrastructure, there is no solution forthcoming over how to prevent it.
The backstop was conceived as a guarantee the Irish border would remain open after Brexit.
Although the UK wants a free trade agreement with the EU post-Brexit, the backstop would apply in the event that an alternative solution cannot be found before the end of the transition period.
It envisages the UK remaining in a "single customs territory" with the EU, with Northern Ireland more deeply integrated with the bloc's rules.
The idea of the backstop was rejected by Northern Ireland's Democratic Unionist Party (DUP), which props up the UK government in parliament, and hardline Brexiters.
The DUP fear being separated from the rest of the UK and so don't like the idea of being in a different regulatory regime to London.
Brexiters, meanwhile, fear the UK will stay in the EU indefinitely if they sign up to a backstop without a time limit.
"We would like a deal, but the backstop, certainly in its current form, is undemocratic and it's something that will have to be removed," said UK foreign secretary Dominic Raab on Thursday.
People cross the 499-kilometre frontier between Ireland and Northern Ireland every day.
Northern Ireland's Department for the Economy estimated in a September 2018 [report](https://www.economy-ni.gov.uk/sites/default/files/publications/economy/Cross-Border-Movements-Research-Article-19-Sep 2018 FINAL.pdf) based on mobile phone data that there are approximately 105 million border crossings per year.
An estimated 72 million vehicles cross per year, according to the Northern Ireland Statistics and Research Agency.
Euronews spoke to a mother and daughter who are from a small border town called Clones. To get to the closest hospital, the road crosses the Irish border four times.
"We wouldn't even be conscious now of when we're crossing," said Mary who is a student at Trinity College Dublin.
In May 2019, the Republic of Ireland sold €169 million worth of goods to Northern Ireland, according to the Rep. of Ireland's Central Statistics Office. Ireland imported €123 million worth of goods from Northern Ireland that same month.
The Republic of Ireland is Northern Ireland's biggest trading partner for both imports and exports.