The Supreme Court is hearing oral arguments over Obamacare today. Here’s what that means.

The Supreme Court on Wednesday will hear oral arguments in King v. Burwell, the most serious challenge to the Affordable Care Act (a.k.a. Obamacare) since the justices upheld it as constitutional almost three years ago. Here’s how this works:

What’s the central issue in King v. Burwell?

The Affordable Care Act provides federal subsidies to low- and middle-income people who buy health insurance through individual marketplaces, or exchanges, in each state. The federal government sets up exchanges in states that do not do it for themselves. The law says the subsidies are available to those who buy insurance through an exchange “established by the state.” The IRS said that in the context of the entire law, that applied to exchanges in the state set up by the federal government. But challengers say that language means subsidies don’t apply to those exchanges set up by the federal government.

Find out more on the subsidies from Jason Millman here.

So why did the Supreme Court decide to take this case? 

Generally, the justices take a case if lower courts have interpreted the law differently. Here a panel of the U.S. Court of Appeals for the D.C. Circuit ruled for challengers, and a panel of the U.S. Court of Appeals for the 4^th Circuit ruled for the Obama administration. Although the full D.C. circuit nullified its panel decision so all the judges could weigh in, the Supreme Court did not wait for the outcome.

Lawyers will make their cases and answer questions from the justices today. How much do oral arguments affect the justices’ rulings?

Justices say the arguments are a time for them to question the legal reasoning advanced by the parties in their briefs. The justices say that sometimes, but not often, oral arguments will affect the outcome of the decision.

What else do the justices consider?

They consider the court’s precedents on the issue, their own theories of constitutional and statutory interpretation, and friend-of-the-court briefs submitted by interested parties. But, as Justice Ruth Bader Ginsburg has said, the case is most often decided based on the written briefs submitted to the justices.

What will we know at the end of arguments on March 4?

The justices sometimes reveal their thinking on the case through the kind of questions they ask the lawyers in front of them. It is also the first time the justices discuss the case themselves, so the way they ask questions is sometimes a signal to the rest of the court.

 What happens next?

The court will meet on Friday and vote on the case. This will probably determine the outcome, though the public may not know the result for several months.

Well, when will we know? 

A case as important as this one tends to take some time. The ruling will be issued once the majority has agreed upon an opinion and, if there are dissenters, when those opinions are ready as well. The court’s only deadline is that it tries to finish its work by the end of June.

Didn’t the justices already hear a case about the Affordable Care Act?

In 2012, the justices ruled 5 to 4 that act was constitutional — meaning that Congress did not exceed its authority in passing it. Chief Justices John G. Roberts Jr. wrote the opinion, joined by the liberal justices Ginsburg, Stephen G. Breyer, Sonia Sotomayor and Elena Kagan. Justices Antonin Scalia, Anthony M. Kennedy, Clarence Thomas and Samuel A. Alito Jr. would have found it unconstitutional.

Does that case affect the King v. Burwell case?

It shouldn’t. The previous decision found the act constitutional. This case is about how to interpret some of the wording in the law.

Which justice or justices might cast the deciding votes?

Since Roberts cast the deciding vote last time, he will be most closely watched this time. Some contend that Kennedy and Kagan might also be “in play.”

Who writes the opinions?

How many can there be? If Roberts is in the majority, he can choose to write the opinion or assign it to someone else in the majority. If he is not on the prevailing side, the senior justice in the majority gets that option.  A justice who agrees with the outcome but not the legal reasoning of the decision may write what is known as a concurring opinion. Dissenters may also write for themselves.