Why do politicians keep talking about locks?

But although the political “lock” appears here to stay – is there a danger that the metaphor’s power is beginning to wane?

“It feels like the less people trust politicians’ promises, the more they have to up their rhetoric when promising things,” says Robert Colvile, the boss of the Centre for Policy Studies think tank and co-author of the 2019 Tory manifesto.

He likens the logic behind the trend for “locks” to the so-called EdStone – Ed Miliband’s much-pilloried attempt to win trust for his key pledges at the 2015 election by engraving them on a large stone tablet.

“The problem is that the more people use this language, the less convincing it becomes,” he adds.

It’s a sentiment shared by Karl Pike, who warns that over-use could mean the phrase starts to “lose meaning”.

He adds that lock has become the “current rhetoric” within politics, but New Labour was fonder of a different metaphor, the “contract”, which could even have its time in the sun again ten years from now.

“Politicians do latch on to particular kinds of phrases,” he adds.

“Whether they’ve been tested or not, whether they are effective or not, they’ve decided they have a kind of political currency”.