Since last month’s general election, there’s been much talk about different types of constituencies, how they voted, and what the geography of party support tells us about how the country voted.

But it also matters for how the next electoral map will be drawn. Among the many consequences of the Conservatives winning a clear majority is that the decade-long standoff over boundary changes will be resolved. But why have boundary changes been controversial?

It’s an accepted principle of our electoral system that constituency electorates should be approximately the same size. And since electorates change over time, constituency boundaries need to be redrawn periodically in order to maintain something close to parity.

Where the process becomes party political is in the specifics of maintaining parity. Changes in constituency electorates over time don’t occur randomly, but have followed a consistent pattern since the war.

Electorates in deprived, generally urban (and Labour voting) areas tend to fall over time, and those in more affluent, generally non-urban (and Tory voting) areas tend to rise. That means that without boundary changes, the number of Labour MPs increases relative to the electorates they represent, while Conservative constituencies tend to become under-represented.

At the election, for example, the average Labour constituency’s electorate was around 3,800 smaller than the average Tory constituency. As such, the Conservatives always prefer for boundary changes to happen as regularly as possible, and stick more strictly to size quotas, while Labour would prefer them to happen less regularly and with more leeway to avoid splitting council areas.

The current difference in size is particularly large as the electorates in use are now almost 20 years out of date. But the disparity in seat size has also favoured Labour at every election since 1959.