The "one person, one vote" doctrine—which says that political districts should contain roughly the same number of people—got a big boost today (April 4).

The Supreme Court just handed down a decision in Evenwel v. Abbott, a case out of Texas that challenged the court’s interpretation of "one person, one vote." Currently, congressional districts are created so that each district has an equal number of people. Officials use population numbers drawn from the U.S. Census Bureau, which attempts to count all of the people residing in the United States every ten years.

The question at issue in this case was whether it is fair for people who aren’t eligible to vote—including children, noncitizens and people who have lost their voting rights due to incarceration—to be counted in the population numbers that shape these districts. Texas State Republican Executive Committee member Sue Evenwel and her co-plaintiffs argued that the spirit of the doctrine is to count only eligible voters.

The Supreme Court disagreed via a rare unanimous decision written by Justice Ruth Bader Ginsburg. In it, she writes:

As the framers of the Constitution and the Fourteenth Amendment comprehended, representatives serve all residents, not just those eligible to vote. Nonvoters have an important stake in many policy debates and in receiving constituent services. By ensuring that each representative is subject to requests and suggestions from the same number of constituents, total-population apportionment promotes equitable and effective representation.

Any other decision would have had major repurcussions for communities of color in the United States, who are more likely to be non-citizens, formerly incarcerated or under the age of 18 than their White counterparts. As Vox explains:

But the reason that the case was so important—and why it’s so surprising the Court’s ruling was unanimous—is that it’s also a debate about the relationship between demographics and political power.

After all, some groups are more likely to be eligible to vote than others. Nearly 80 percent of White people in America are citizens over the age of 18; fewer than half of Hispanics in America fit that description.

In practice, then, carving up congressional districts based on eligible voters would result in fewer representatives for heavily Hispanic and Asian parts of the U.S. And, unsurprisingly, the seats it would have gotten rid of would have mostly been Democratic.

The Court has not yet issued a ruling on the other "one person, one vote" case it heard last December. In Harris v. Arizona Independent Redistricting Commission, a group of Arizona voters argued that the commission created legislative maps that violated the principle and bolstered the population in the state’s Republican-leaning districts.