In the US, every state elects a certain number of people to the House of Representatives — a number that’s based on the Census count of the state’s population. North Carolina, for instance, elects 13 House members. So North Carolina has to be divided into 13 congressional districts with roughly equal populations. In most US states, this process is controlled by the majority party in the state legislature.
Partisan gerrymandering occurs when this map-drawing process is intentionally used to benefit a particular political party — to help that party win more seats, or more easily protect the ones it has. The goal is to create many districts that will elect members of one party, and only a few that will elect members of the opposite party. You can see North Carolina’s Congressional district map below:
You’ll notice that’s not a very clean map. It’s full of bizarre shapes, weird outcroppings and sharp turns. That’s no accident. The map was drawn by North Carolina’s Republicans, and it did its job. Though Democrats won around the same amount of votes as Republicans in the state’s 2018 House races, the GOP hung on to an unchanged 10-3 majority in the state’s congressional delegation. Not a single seat flipped.
Gerrymandering can affect any legislative body that has to have districts drawn — which includes both the US House of Representatives, and every state legislature. Both parties have tended to do it when the opportunities arise. And since political power is at stake, fights over redistricting are often quite intense.
The term gerrymandering is also sometimes used to describe somewhat different redistricting scenarios. Racial gerrymandering can mean the dilution of the voting power of certain racial or demographic groups, which is usually entangled with seeking partisan advantage. And a bipartisan gerrymander is a redistricting meant to protect incumbents of both parties.
The story of how gerrymandering got its name is actually pretty interesting. You can read it here.