Stephanopoulos and McGhee argue that fair districting requires a roughly equal number of wasted votes for each party, and that districting schemes where one party is wasting many more votes are unconstitutional. They call their metric the “efficiency gap,” calculated by taking the difference between the number of “wasted votes” for each party, and dividing that difference by the total number of votes.

The efficiency gap is key to the plaintiffs’ arguments in Gill v. Whitford. They proposed setting a threshold of 7 percent: If a districting plan produces a larger gap than that, if one party is getting a wasted-vote advantage of more than 7 percent of the total vote, then it’s getting a huge leg up, which will continue for a long time. As Yale Law School dean Heather Gerkennoted in a Vox piece following the initial district court decision, a gap above that amount indicates that the disadvantaged party “would have almost no chance of taking control of the legislature during the 10-year districting cycle.”

By contrast, the Wisconsin plan created efficiency gaps of 13 percent and 10 percent in 2012 and 2014, respectively. Those are truly massive advantages enjoyed by the Republican Party.

By taking up the case, the Supreme Court is essentially promising to rule on the merits of the efficiency gap as a means of determining whether an improper partisan gerrymander has happened — and, if one has occurred, on whether that violates either First or 14th Amendment protections.

Read more at Vox