Lines on maps matter—to security, to identity and to elections. In 2023 the boundaries of Britain’s 650 constituencies were revised for the first time since 2010. Unlike redistricting in America, the exercise in Britain is determinedly non-partisan. Its effects are not. New data released on January 16th cast some light on how these new boundaries will affect MPs’ prospects of re-election.

With a few exceptions the new boundaries follow a simple rule, laid down in 2020: to keep the number of electors in each constituency within 5% of 73,393, while staying faithful to geography. The process was run by independent boundary commissions in each of Britain’s four devolved nations. Political parties took part. But Glenn Reed, from the Boundary Commission for England, asserts that the consultations gave as much weight to the views of Joe Public as to, say, the Conservative Party’s campaign headquarters. He says the borders of one constituency in Devon were amended on the back of one individual’s persuasive argument at a town-hall meeting.

The changes are substantial. Greater absolute population growth in the south of England means that this region will have 15 more MPs at Westminster after the next general election. Wales will have eight fewer; Scotland two fewer. The equalisation rule means that most constituencies’ borders have been altered. Only 65 were spared the geographers’ red pen.

Outside Northern Ireland, where the differences from 2010 are marginal, the population of electors in the remaining 554 seats is, on average, 20% different from those inside the old boundaries. Fully 265 old constituencies have been split into entirely new areas. The 63,000 former constituents of Ogmore in south Wales, for example, have been scattered among five newly created constituencies, from Cardiff to the Rhondaff.

To see how the new boundaries affect parties’ prospects, psephologists have tried to work out how each of them would have fared if the boundaries had applied at the last general election, in 2019. Colin Rallings and Michael Thrasher, both professors of politics at the University of Plymouth, have just published such estimates on behalf of Britain’s three largest broadcasters.

The pair calculate that the Tories would have won seven additional seats in 2019 under the new boundaries, taking their total to 372 (see chart). The allocation of more seats in the south of England means they have notionally “gained” 11 there. The Labour Party, whose 202-seat total in 2019 marked its worst election performance since 1935, would have won two fewer seats overall.

That appears to mean that the changes make it harder for Labour, now led by Sir Keir Starmer, to achieve a majority. Messrs Rallings and Thrasher calculate that whereas the party would have required a 12.0 percentage-point swing from the Conservatives under the old boundaries, it now needs a 12.7-point swing. That is equivalent to winning 4m more votes than it managed in 2019.

This is unlikely to dismay Sir Keir too much. According to The Economist’s tracker, the latest opinion polls imply a 14-percentage-point swing to his party since 2019, and Labour’s vote will be more efficiently spread than at that vote. A large poll released by YouGov this week suggested that the Conservatives would lose 196 seats if the election were held tomorrow. That would be their worst result since 1997. The new boundaries may have given Tory MPs a little extra edge. It won’t make them less twitchy. ■