The US Supreme Court has ruled that they cannot decide in cases of alleged partisan gerrymandering. That gives the green light to politicians to draw electoral boundaries in such a way as to favour their party.

Although different rules apply in the different states, in simple terms electoral boundaries in the US, particularly congressional districts, are drawn by state legislators based on the most recent census. The numbers in each district have to be roughly equal but the boundaries are up to local decision making.

Why does it matter how the boundaries are drawn? Because each community is different and people of like mind tend to be more likely to live near each other. So you have areas which are more Republican and areas which are more Democratic. Drawing the boundaries in a different way can skew the results of an election. The Electoral Reform Society has produced a graphic which neatly illustrates this problem.

The US has long had a history of politically partisan boundary delimitation. The term gerrymandering was created there. The team at FiveThirtyEight have come up with a number of different examples of how the extent of gerrymandering could affect the US House of Representatives.

There are five states which have put in place rules to (largely) protect against partisan boundary drawing. But others leave it up to their legislators. Two such are North Carolina and Maryland and these were the cases before the Supreme Court whose decision was made public today.

This is not the end of the matter. First state courts might choose to get involved in the issue and have the right to do so even though the Supreme Court has spoken. The Pennsylvania Supreme Court ruled last year that the congressional map drawn by the Republican legislature in 2011 ran foul of the state constitution, saying that the map violated the state’s guarantee of “free and equal” elections. They chose to redraw the map themselves. As many as a dozen other states have similar provisions in their constitution. In addition, states can pass their own laws that would require boundaries to be drawn in a non-partisan manner, either through their own legislature or through a referendum.

But in the short term this (non) decision by the Supreme Court could open the floodgates for more state legislatures to take a radical view of boundary drawing.