It has been called a textbook example of discrimination against Black voters in the US. And a ruling on it from the supreme court is expected any day.

It isn’t the kind of explicit voting discrimination, like poll taxes and literacy tests, that kept voters from the polls in the south during the Jim Crow era. Instead, it is more subtle.

Let us walk you through the case with our visual explainer.

The case focuses on Alabama, where the Republican-controlled legislature, like states across the US, recently completed the once-a-decade process of redrawing the boundaries of congressional maps. If partisan politicians exert too much control over the redistricting process, they can effectively engineer their own victories, or blunt the advantages of the other side, by allocating voters of particular political persuasions and backgrounds to particular districts.

Under the new districts, Black people make up 25% of the Alabama’s population, but comprise a majority in just one of the state’s seven districts.

In late January, a panel of three federal judges issued a 225-page opinion explaining how the state was discriminating against Black voters.

“Black voters have less opportunity than other Alabamians to elect candidates of their choice to Congress,” the panel wrote. The judges gave Alabama 14 days to come up with a new plan and said the state had to draw two districts where Black voters comprise a majority.

Now Alabama is asking the US supreme court to step in and block it from having to redraw its congressional districts. In a brief to the supreme court, lawyers for Alabama argued that by creating two majority-Black districts, they would be prioritizing race over other the state’s traditional redistricting criteria such as keeping communities whole and keeping districts as compact as possible.

The plaintiffs in the case dispute that. In their own brief to the supreme court, they noted that several of their sample maps comply with Alabama’s redistricting criteria – some even more so than the map the state enacted. It is possible to simultaneously have two Black districts and observe these criteria, they say.

There is also an element of common sense involved, say plaintiffs, who point to the fact that the Black belt – named for its rich topsoil – has been an area with a high concentration of Black people for hundreds of years. “You have the Black belt … In Alabama it is such a well-known and unique feature of the state. Everyone knows that’s where Black folks live and have lived for 200 years and are still concentrated,” said Deuel Ross, an attorney at the NAACP Legal Defense and Educational Fund who is representing some of the plaintiffs in the case. “This is a community that has been together.”

The supreme court ruling could have big consequences for section 2 of the Voting Rights Act (VRA), a crucial part of the landmark 1965 law that prohibits any voting practice that discriminates on the basis of race, and which is commonly used to bring challenges like the one in Alabama. If the justices side with Alabama, it would probably make it very difficult to bring similar future challenges under section 2.

Michael Li, a redistricting expert at the Brennan Center for Justice, said Alabama was taking a “swing for the fences” argument to try to take down a “major part of the edifice of the Voting Rights Act”.

“It’s really a very aggressive stand,” he said. “What Alabama really is saying is the VRA has to be subordinate to the state’s neutral criteria. And if you can’t draw a VRA district while complying with the state’s neutral criteria, there’s no liability, which would be a big change.

“It subordinates federal law to state law. States then could define all sorts of neutral criteria that would make it impossible to ever draw a VRA district,” he added.