Image copyright Getty Images Image caption How much time has to elapse before there's a new Yes generation?

Boris Johnson has rejected Nicola Sturgeon's call for a new independence referendum by insisting the 2014 vote was a "once in a generation" event. Why do people keep using this phrase, and what does it mean?

Why is this coming up now?

The debate about a second independence referendum has echoed much of the debate about the first one, because both sides see it as setting a useful precedent.

First Minister Nicola Sturgeon wants the UK government to agree a transfer of powers to hold a vote, because that was the "gold standard" process used in 2014.

Boris Johnson doesn't want another referendum, and is using some of Ms Sturgeon's own words from the original campaign as justification for saying no.

The prime minister's letter rejecting Ms Sturgeon's request told her "you and your predecessor made a personal promise that the 2014 independence referendum was a 'once in a generation' vote. The people of Scotland voted decisively on that promise to keep our United Kingdom together."

He has echoed this line whenever questioned about the matter, but the SNP insists times have changed and the party has an "unarguable" mandate for a new referendum.

Who said what?

In Alex Salmond's foreword to Scotland's Future, the white paper on independence ahead of the 2014 referendum, the then first minister said the vote would be a "rare and precious moment in the history of Scotland - a once in a generation opportunity to chart a better way".

The document itself went on to state there was "no arrangement in place for another referendum on independence", and that "only a majority vote for Yes would give certainty that Scotland will be independent".

Mr Salmond and his team used the phrase several times in interviews, including one with Andrew Marr where he said that "in my view this is a once in a generation, perhaps even a once in a lifetime opportunity".

His deputy Ms Sturgeon, used that "lifetime" phrase during the final Holyrood debate before the referendum - as did three other SNP MSPs, before passing a motion which also called the ballot the "opportunity of a lifetime".

Media playback is unsupported on your device Media caption Alex Salmond says the Scottish referendum is a "once in a generation opportunity"

Independence supporters contend Mr Salmond was stating his own view - he told Mr Marr that "it is just my opinion" - and that the position of his government cannot bind that of its successors.

The SNP also points to the Smith Commission report on extra powers for Holyrood, published two months after the referendum, which stated that "nothing in this report prevents Scotland becoming an independent country in the future should the people of Scotland so choose".

How long is a generation anyway?

If you're an aphid of the rhopalosiphum prunifolia family, a generation is apparently 4.7 days.

For humans, an essay for the International Society of Genetic Genealogy seems to have narrowed the length of the average generation down to somewhere in the range of 29 to 34 years.

But how long is a political generation?

This becomes a bit like speculating on the length of a piece of string. Scotland Office minister Douglas Ross took the scattergun approach of saying it should be "30, 40, or 50 years" in an interview with the BBC's Good Morning Scotland programme on Wednesday.

His boss Alister Jack, the Scottish secretary, had earlier suggested a vote shouldn't take place during Ms Sturgeon's lifetime - which based on the average life expectancy for women of her age would be at least another couple of decades yet.

Image copyright Getty Images Image caption The Good Friday Agreement suggested seven years was a reasonable gap between referendums

Are there any historical examples which might help?

There were 18 years between the two referendums on Scottish devolution, in 1979 and 1997. Mr Salmond cited this gap in that Andrew Marr interview, saying "that's what I mean by a political generation".

Meanwhile there were 41 years between the referendums on EU membership in 1975 and 2016, a stretch pointed to by interim Scottish Conservative leader Jackson Carlaw.

But shifting to another constitutional setting, the Good Friday Agreement stipulates that there should be seven years between border polls on Irish reunification.

That figure is actually set out in legislation. Obviously the context of political history in Northern Ireland is quite different to that of Scotland, but the deal containing that seven-year provision was endorsed both by MPs and, obviously, in a referendum.

Is it just a turn of phrase?

The SNP contend that it was never their intention to shut the door on future votes - why would they? - and that the phrase was more a way of expressing how crucially important they thought the 2014 referendum was.

They say it wasn't a "personal promise", as the prime minister claims, but the sort of rhetoric that politicians employ every day.

Context can be key.

For example, Mr Johnson said December's general election was a "critical, once in a generation" vote - but nobody really expected it to be the last Westminster poll for decades.

Image copyright PA Media Image caption There are few topics on which Nicola Sturgeon and Boris Johnson look eye to eye

The Tories say expectation levels are different for referenda - they are designed to settle binary questions, and happen far less frequently. The last thing we want, they say, is a "neverendum".

Although on that latter point, there has been no shortage of referendums since the Conservatives came to power in 2010 - on the electoral system, on Scottish independence, on increased powers for the Welsh Assembly, and on Brexit.

Do things change with time?

The chief argument employed by the SNP for a new referendum is that things have changed substantially since 2014. The party says the UK that was talked about at the time, which Scots voted to remain part of, no longer exists.

The Brexit vote is the key driver of this, given the 62% backing for Remain in Scotland - and it was the No campaign who insisted in 2014 that "Scotland enjoys membership of the EU because of our membership of the UK".

The Conservative rebuttal to this would be that the UK as a whole voted to leave the EU - this was a change mandated by the people, and thus does not itself justify another change.

Of course there have also been a slew of other votes since the referendum.

In another interview in 2014 Mr Salmond said "the only circumstances in which you could have another referendum would be if you got an extra mandate at a subsequent general election".

The SNP have come out on top in every election north of the border since then, most recently taking 45% of the Scottish vote in the general election - a bigger share than Mr Johnson's triumphant Conservatives got UK-wide.

If that kind of form keeps up - particularly at the Holyrood election in 2021 - the prime minister may need to find a new catchphrase.