What exactly is happening on Friday?

It is the second reading of the terminally ill adults (end of life) bill, which would make assisted dying legal for terminally ill adults who are expected to die within six months and have the mental capacity to make a choice about how they end their life, subject to tough safeguards.

Slightly confusingly for non-parliamentarians, the second reading of a bill is the first chance that MPs get to debate and vote on it – the first reading almost always involves just a literal reading of the bill’s title to the Commons.

On Friday, MPs will have five hours to debate the bill, followed by a vote due at 2.30pm. A total of 160 MPs have said they want to speak – but there will not be time for all of them.

What is special about this bill and the process?

It is a private member’s bill, the name given to proposed laws introduced by individual MPs rather than the government. Normally these are not given enough parliamentary time to have a chance of passing. But after every state opening of parliament, a ballot is held for MPs to be given priority for their bills.

Kim Leadbeater, the Labour MP who is behind the assisted dying bill, came first in the ballot in September, and chose it as an issue. As it is not a government bill, ministers are officially neutral, and none will speak on Friday. The vote will be free – that is, MPs will not be instructed by their parties, and will decide for themselves which way to vote.

What will happen next?

It very much depends on the results of the vote. If MPs vote against the bill, it is finished, with the idea of a change to the law on assisted dying most likely put off again for years – before Leadbeater’s bill, the last attempt was in 2015.

If it passes, the bill goes to the next parts of the Commons process, the committee and report stages, where there is considerably more time for scrutiny and the opportunity for amendments to be tabled. This would not begin before next April. It would then be put to a third reading vote in the Commons before, if it again passed, going to the Lords.

While it would remain a private member’s bill, it is expected that government departments would start impact assessments and also examine the viability of any amendments. The committee stage involves MPs going over the measure.

Leadbeater has promised to include opponents, while a Ministry of Justice minister would be assigned, who could recommend amendments to adopt.

Why is the bill being done in this way?

Traditionally, major social changes have come through private member’s bills, perhaps most famously the 1967 act to legalise abortion, put forward by the Liberal MP David Steel.

Assisted dying is a highly contentious issue, and one which touches on very personal issues including faith and family history. A government bill would need to be supported by ministers, and some very senior cabinet members – among them the health secretary, Wes Streeting, and the justice secretary, Shabana Mahmood – vehemently oppose it.

Will it pass?

No one knows – genuinely. A lot of MPs remain publicly undecided, and there is no government whipping operation to tally the numbers.

Proponents of the bill are trying to stress to uncertain MPs that voting for it on second reading is not an unconditional yes, just a green light to more scrutiny. But it could be very close.