On Friday, the U.S. Sentencing Commission voted unanimously to allow nearly 50,000 nonviolent federal drug offenders to seek lower sentences. The commission's decision retroactively applied an earlier change in sentencing guidelines to now cover roughly half of those serving federal drug sentences. Endorsed by both the Department of Justice and prison-reform advocates, the move is a significant step forward—though in a global context, still modest—in reversing decades of mass incarceration.

How large is America's prison problem? More than 2.4 million people are behind bars in the United States today, either awaiting trial or serving a sentence. That's more than the combined population of 15 states, all but three U.S. cities, and the U.S. armed forces. They're scattered throughout a constellation of 102 federal prisons, 1,719 state prisons, 2,259 juvenile facilities, 3,283 local jails, and many more military, immigration, territorial, and Indian Country facilities.