Plans to trigger another referendum on independence have been announced by Scotland's First Minister Nicola Sturgeon.

Hang on. Didn't Scots just have a referendum on independence in 2014?

Scots voted by 55% to 45% to stay part of the UK in September 2014. Despite that vote, the issue is far from resolved.

Following the referendum, the SNP saw a massive surge in membership, with the nationalists now the third biggest political party in the UK. More evidence of the party's popularity came in the 2015 general election when they won all but three of the 59 seats up for grabs north of the border.

READ MORE: First Minister Nicola Sturgeon plans second independence referendum

In 2016 the party won a third term in power at Holyrood, although Nicola Sturgeon lost the overall majority her predecessor Alex Salmond had won in 2011.

However, the SNP manifesto for the May 2016 Scottish elections stated: "We believe that the Scottish Parliament should have the right to hold another referendum if there is clear and sustained evidence that independence has become the preferred option of a majority of the Scottish people - or if there is a significant and material change in the circumstances that prevailed in 2014, such as Scotland being taken out of the EU against our will."

Just weeks after the Scottish Parliament elections, the European Union referendum saw 62% of Scots vote to remain while the UK as a whole voted to leave - the very situation outlined in the SNP manifesto.

Within hours of the result being known, Ms Sturgeon warned a second independence referendum was "highly likely".

So when would a second referendum be held?

It is not clear when another ballot would take place and the timing would most likely have to be the subject of agreement between the Scottish and UK governments, in a similar way to the Edinburgh Agreement, which outlined how the 2014 referendum would be held.

Ms Sturgeon has already indicated the autumn of 2018 could be a "common sense" time for a referendum, but there has been speculation that Downing Street may attempt to delay another vote until after Britain has exited the EU.

Would the legislation for a second independence referendum be passed by Holyrood? And what about Westminster?

While the SNP lost its majority in 2016, winning 63 of the 129 seats in the Scottish Parliament, the pro-independence Scottish Green Party increased its tally of MSPs from two to six, meaning there is a majority of MSPs in Holyrood who support independence.

The Scottish Government has already carried out a consultation on a draft Referendum Bill, which sets out how a second vote on independence could be held.

Theresa May has not yet said if the Conservatives would seek to prevent a referendum from being held, by voting against a section 30 order to transfer the necessary legal power for the ballot. Instead, the Tories say simply that a second referendum should not be held.

However, a number of senior politicians at Westminster, including Labour leader Jeremy Corbyn, have said that Westminster should not seek to block a fresh vote on independence.

What if Scots vote to stay part of the UK again, would Nicola Sturgeon have to resign?

Within hours of the result of the 2014 referendum result being known, Ms Sturgeon's predecessor Alex Salmond had announced he would be stepping down as both SNP leader and Scotland's first minister.

While it seems certain that a second vote against independence would mean there could not be another such ballot for a considerable period of time, it is not certain that Ms Sturgeon would have to step down.

Politics expert Professor James Mitchell, of Edinburgh University, told the Press Association: "It's not as certain as some imagine that she (Ms Sturgeon) would go, that would depend on how she handles defeat.

"I think she could get away with staying on in a way he (Mr Salmond) could not have."

READ MORE: First Minister Nicola Sturgeon plans second independence referendum

He argued Mr Salmond had been SNP leader for 20 years - over two spells each lasting a decade - whereas Ms Sturgeon has, in comparison, been head of the party for less than three years, taking over the role in November 2014.

In addition, Prof Mitchell said there was no obvious successor to Ms Sturgeon waiting in the wings in the party.

He said: "There's no rule she would have to go, for all sorts of reasons I don't think it would be certain."