Though the boundaries of our parliamentary constituencies are redrawn regularly, there’s enough continuity to allow for the existence of what are known as bellwethers: seats whose result time-and-again mirrors exactly the overall result of the general election. In other words, whichever party wins the bellwether seat also goes on to form the government.

Whether you think this sort of thing is just pure coincidence or a plausible model for predicting an election, bellwethers have been a feature of UK politics for decades. The media loves them, and understandably: they give the business of covering an election an intriguing and localised dimension, besides introducing a bit of predicability into the otherwise imprecise art of forecasting the winner.

I’ve counted four bellwethers that will apply in this year’s general election, though there may be more.

Since its creation in 1983, Reading West has always returned an MP from the same party that forms the government. It’s been represented since 2010 by Alok Sharma for the Conservatives. Labour needs a 6.3% swing to take it back.

Bristol North West is the next oldest. It has been a bellwether since October 1974. Labour needs a 6.1% swing to take the seat this year, after coming third in 2010 behind the Tories and the Lib Dems.

Loughborough has been a bellwether since February 1974, and currently has education secretary Nicky Morgan as its MP. Labour needs a 3.6% swing to win here.

Dartford is the oldest bellwether. In every election since 1964 it has returned an MP from the same party that has gone on to form the government. A 10.6% swing to Labour would see it change hands.

Bear in mind bellwethers may have limited value this year, especially if the two main parties end up receiving under two-thirds of the total votes cast.

Indeed, all four of the above seats would lose their status as bellwethers if the Tories hold them (which seems likely) but Labour ends up in government at the head of a coalition (which also seems likely). Decades of tradition and novelty value would come to an end, just like that.

As a footnote, two long-running bellwethers have lost their status in recent years. Basildon was one of the most famous bellwethers from its creation in February 1974 to its abolition in 2010, when it became part of Basildon and Billericay. Chorley is another ex-bellwether, picking an MP from the governing party in every general election from 1970 until 2010. Meanwhile Corby was a bellwether from 1983 to 2010. Labour won the seat in a by-election in 2012, and if the party holds it in 2015, it will retain its bellweather credentials.

UPDATE: As Andrew points out below, Watford has been a bellwether since February 1974. Labour lost the constituency in 2010 after coming third behind the Tories and the Lib Dems. The party needs a swing of 4.1% to retake the seat.