The Supreme Court will answer a key question about the meaning of "one person, one vote."

Legislative districts must have roughly equivalent populations. That's been Equal Protection law for a long time. But how do you count the population? Do you include all residents or just those who are eligible to vote � or are states free to use either count?

Almost all state and local governments draw districts based on total population. If people who were ineligible to vote were evenly distributed, the difference between counting all people or counting only eligible voters would not matter. But demographic patterns vary widely.

If the challengers succeed, the practical consequences would be enormous, Joseph R. Fishkin, a law professor at the University of Texas at Austin wrote in 2012 in The Yale Law Journal.

It would, he said, �shift power markedly at every level, away from cities and neighborhoods with many immigrants and many children and toward the older, whiter, more exclusively native-born areas in which a higher proportion of the total population consists of eligible voters.�

"One person, one vote" sounds like a reference to the voters, but that phrase comes from the court cases, not the Constitution itself. You could also think in terms of equality in the number of persons each representative represents. And if we're going to think in terms of voters, why would we look at the number of persons eligible to vote as opposed to the number of persons who actually vote? We know that voter turnout varies geographically.