The United States Sentencing Commission just voted to let 46,290 federal prisoners apply to get out of prison sooner. The vote applied the Sentencing Commission's latest reductions in federal sentencing guidelines, approved in April, to people currently serving sentences in federal prison for drug crimes.

Prisoners will begin to be released on November 1, 2015.

How many prisoners does this affect?

The Sentencing Commission estimates that 46,290 prisoners will be eligible to apply for shorter sentences under the new guidelines. That's nearly half of all federal drug offenders — but when it comes to all prisoners in the US, it's just a drop in the bucket.

A little over 20 percent of all inmates in federal prison are going to be able to apply for shorter sentences. (Nearly half of all federal prisoners are locked up for drug crimes, and nearly half of all federal drug prisoners are going to be able to apply for shorter sentences under this change.)

But most prisoners in the US are doing time in state prisons, not federal ones. Looking at the whole prison population of the US — at all levels — this change only affects about 3 percent of all prisoners in America.

What kind of crimes did these prisoners commit?

This plan applies to prisoners who've been convicted in federal court of possession or distribution of drugs.

In April, the Sentencing Commission voted to change the way federal sentencing guidelines are calculated in drug cases, so that the quantity of drugs someone is caught with isn't as important a factor in determining how long his sentence is. Policymakers and law enforcement professionals thought that drug quantity isn't actually as important in deciding how serious a crime is than other factors, so they changed the way judges calculated recommended sentences to reflect that. Those new rules will go into effect on November 1st, 2014 (unless Congress acts to stop them).

Today, the Sentencing Commission agreed that it wasn't fair that prisoners who've already been sentenced have to serve more time under the old rules, while people who commit crimes in the future will get to serve less time under the new rules. So they voted to make their changes to the federal guidelines retroactive.

How will a prisoner be able to get a shorter sentence?

Prisoners will have to apply to get their sentences reduced, and each application will be reviewed, individually, by a federal judge. (One of the reasons that the Sentencing Commission decided not to start releasing prisoners until November 2015 was to give federal judges more time to work through the applications they're going to receive.) The judge is responsible for deciding whether or not releasing the applicant would be dangerous for public safety, and whether the applicant deserves to have his or her sentence reduced.

Before the judge looks over an application, a federal probation officer will go through the process of calculating what the current federal guidelines recommend for the prison sentence — just as if the prisoner were being sentenced all over again. (The applicant's lawyer and the federal prosecutor will also discuss a suitable sentence recommendation.) The judge will then decide if the prisoner ought to qualify for sentence reduction. If the prisoner does qualify, the judge will pick a sentence from within the recommended range — and account for the amount of time the prisoner's already served.

The Sentencing Commission estimates that the average sentence reduction will be about 25 months — over two years. Eligible prisoners would end up serving an average of 108 months, which is about 20 percent shorter than they were originally sentenced to serve.

How many prisoners will be released immediately?

None. The Commission's plan doesn't allow any prisoners to be released before November 1, 2015. Judges will be able to approve applications for sentence reduction before then, but the actual date of release can't be any sooner than next November. That gives federal judges time to work through all the applications they're likely to receive, and gives the federal government time to put together prisoner reentry programs (like halfway houses) to accommodate the increase in newly-released prisoners.

Is this happening because the Obama administration is pushing to roll back the drug war?

No. The US Sentencing Commission is independent of the Obama administration.

In fact, the Department of Justice originally wanted the Sentencing Commission to approve a much more limited plan — one that would only let about 20,000 prisoners apply for shorter sentences. This week, reports surfaced that Department of Justice officials had been meeting privately with the Sentencing Commission, and had softened its position a little: it now wanted something that would affect about 40,000 prisoners. It's not clear if the plan the Sentencing Commission approved today is the one the DoJ was lobbying for in private, or a different one.

It's actually surprising that the Department of Justice was interested in making any sentences retroactive at all. In 2011, the Sentencing Commission decided to allow current prisoners sentenced for crack cocaine to apply for shorter sentences after Congress reduced the disparity between crack and powder cocaine — but the Department of Justice didn't take a position one way or the other. (Under Bush, the Department of Justice actively opposed sentencing reductions.)

Obama administration officials from Attorney General Eric Holder on down have publicly said that they think too many people are in prison for too long. So why aren't they more supportive of plans to reduce sentences?

One reason is ideological: many law enforcement officials and policymakers believe that changing sentences after a judge has handed them down undermines the rule of law, which might embolden more people to commit crimes. The other reason is political. Federal prosecutors oppose any reduction in criminal sentences, and federal prosecutors are DoJ employees. Attorney General Holder has been trying to encourage prosecutors to warm up to criminal justice reform, but the Department's recommendations to the Sentencing Commission might have been made with them in mind.

Does this do the same thing as the criminal-justice-reform bills that are up in Congress?

No.

The guidelines aren't the only thing that determine how long a criminal sentence is. Some crimes also have "mandatory minimum sentences" — laws passed by Congress that require a defendant to serve at least a certain amount of time, like 5 or 10 years. (Judges can give a defendant a longer sentence than a mandatory minimum requires, but not a shorter one.) The Smarter Sentencing Act, which passed the Senate Judiciary Committee earlier this year, reduces the mandatory minimum sentences, and gives judges more leeway to ignore the minimum sentence in particular cases.

As a matter of fact, the chief judge of the Sentencing Commission said after today's vote that the government should still pass the Smarter Sentencing Act. As it stands now, the reduced sentences that prisoners will start applying for and receiving still can't be any lower than the mandatory minimum for their crimes — even though the Sentencing Commission, the administration and members of Congress in both parties agree that minimum sentence is too long.

Is this going to help reduce the racial disparity of incarceration in America?

That depends on how you look at it.

The majority of people who will be able to apply for shorter sentences are black and Hispanic — the same people who are overrepresented in the American prison population to begin with. The Sentencing Commission's original calculations (which estimated what would happen if people started getting released this November rather than next November, and therefore include slightly more people than the plan the Commission adopted) said that 44 percent of inmates who benefited would be Hispanic, and 30 percent would be black.

Because this change doesn't affect that many prisoners in the grand scheme of things, it doesn't affect the racial imbalance in America's prisons that much either. As of 2012, blacks were 36 percent of the prison population; they account for 37 percent of all prisoners who aren't affected by the new rule.

But for the affected prisoners, the Sentencing Commission's decision today isn't a small thing. Starting next November, 15,600 black prisoners and 22,200 Hispanic prisoners — most of them men — will be able to get out of prison and start their lives again sooner than they had originally thought.