If MPs manage to secure a debate this week on preventing a no-deal Brexit, it will end with a vote – and the result is likely to be very close.
A total of 635 votes will be in play.
The magic number for either side will be 318: enough for a majority of one.
For the Government to win, every Conservative MP eligible to vote (308) would need to be joined by the 10 MPs from the DUP.
This comes to exactly 318.
In reality, on a vote to rule out a no-deal Brexit, MPs are unlikely to divide precisely on party lines.
For example, at least one Labour MP – such as Kate Hoey – could side with the Government and vote against such a proposal.
The former Tory MP Charlie Elphicke, who is currently sitting as an independent, would probably do the same.
In return, a handful of Conservative MPs are likely to do the opposite and vote in favour of the proposal.
Both sides will need to make allowances for defections.
A further twist might come if the independent MP for Sheffield Hallam, Jared O’Mara, resigns this week as a member of parliament.
Mr O’Mara indicated in July that he intended to stand down when parliament returned in September.
If he formally resigns his seat, it would reduce the number of votes in play to 634, besides reducing by one the number likely to vote in favour of legislation to stop a no-deal Brexit.
The 635 votes in play are the 650 MPs in the House of Commons minus seven Sinn Fein MPs (who do not take their seats), the Speaker and three deputy speakers (who do not vote), and the four tellers – two for, two against – whose votes are not included in the overall result.