In February the House of Commons overwhelmingly approved the Article 50 bill, the legislation that sets out the process for the UK leaving the EU.
But the House of Lords then had their say and demanded two amendments to guarantee the rights of EU citizens in the UK and ensure Parliament had a vote on any deal.
Back it went to the House of Commons who will make a decision on those amendments, and then it's back to the Lords again.
The Week
Escape your echo chamber. Get the facts behind the news, plus analysis from multiple perspectives.
SUBSCRIBE & SAVE
Sign up for The Week's Free Newsletters
Join more than 350,000 subscribers and keep yourself informed with a selection of The Week’s most interesting, enlightening and entertaining stories - plus daily puzzles.
Join more than 350,000 subscribers and keep yourself informed with a selection of The Week’s most interesting, enlightening and entertaining stories - plus daily puzzles.
Latest Videos FromThe Week
This process is known as ping-pong. So how does it work?
How a law is made
In the UK, before a draft law, known as a bill, can become legislation it must have been passed by both the House of Commons and the House of Lords.
Most bills are introduced in the House of Commons first, where the main principles of the bill are debated and a vote is cast.
The bill then passes to the House of Lords where peers can make amendments which are then sent – 'pinged' – to the House of Commons. These are either accepted, in which case the bill is passed, or rejected, amended and flung back – 'ponged' – to the Lords.
When does the ping-ponging end?
The bill will ping-pong until both houses agree on the exact wording of the bill. Usually the debates are scheduled weeks or months apart.
No bill has ever bounced between houses more than 15 times but there is no time limit other than the end of the parliamentary year.
Continually playing ping-pong can be time-consuming and if no agreement is reached the bill will fail.
To prevent this, the House of Commons has a 'trump card' that it can play. If, after one year of delay, an agreement has still not been reached, the Commons can use the Parliament Acts to pass the bill without the consent of the Lords.
Historically, the trump card has been played to override the conservatism of the House of Lords, from the Sexual Offences (Amendment) Act 2000, which lowered the age of consent for gay men to sixteen, to the Hunting Act 2004, which controversially banned fox hunting.
What will happen to the Article 50 bill?
The bill is due to be debated between the House of Commons and House of Lords on Monday and time has been set aside on Tuesday and Wednesday in case negotiations stall.
But it looks like the ping-ponging might be short-lived.
David Davis, the Brexit minister, has urged MPs to send the legislation back to the House of Lords in its original form and reject the amendments, reported The Sunday Times.
Meanwhile Vicki Young, the BBC's chief political correspondent, predicts that peers will not block the bill any further.
Despite the pressure being applied by Liberal Democrat peers, it's 'unlikely that the Lords has the appetite to push back on this again', says Norman Smith, assistant political editor on BBC Radio 4's Today programme.
The overwhelming likelihood, reports the BBC, is that the bill will become law and Theresa May will be able to trigger Article 50.
