As soon as Thursday, the Supreme Court could decide the fate of millions of same-sex couples nationwide. In a ruling covering four cases, the court will determine whether states can prohibit same-sex marriage, as 13 states currently do.

It’s always tough to predict how the court will rule but, broadly speaking, there are three main possibilities: the simplest is that the court declares state marriage bans unconstitutional, meaning states will all perform and recognize same-sex marriage. That’s a pretty simple outcome, but things get much trickier in the other two cases.

One other possibility is that the court decides to uphold bans. That means states that currently have bans could continue having theirs. But it also leaves 20 states up in the air legally. That group includes states where federal action struck down state bans. If the Supreme Court says bans are constitutional, those states could go back to having bans in place.

And there’s also the possibility of the court saying bans are constitutional, but that all states must all recognize marriages performed in other states. This option retains the messiness of the above possibility, but it does mean that couples would be recognized equally nationwide.

While you can break the decisions down into three neatly color-coded maps, there is a complicated web of state laws at work, and it means outcomes could vary widely by state if the court decides bans are constitutional. Adam Romero, senior counsel at UCLA’s Williams Institute, says the states where federal action struck down state bans are where things could get really complicated.

“In those 20 states it’s uncertain if the bans would go back into effect because there’s a couple of different things in play,” he says. “So in those 20 states including California, Utah, et cetera, the bans could be reinstated, but there would need to be legal maneuvers done to reinstate the bans.”

And it’s not clear that those legal maneuvers would happen in all of those states. He points to Utah as an example. The court case that struck down Utah’s ban has long been over. If the Supreme Court were to uphold bans, Romero explains, the defendants in that Utah case would have to go back and ask the court to undo its injunction if the state were to have another ban. That could easily happen, but it’s also not 100 percent certain to happen.

In addition, there are complicated legal questions that could come into play for couples in those 20 states if the court decides to uphold bans — if you were married when it was legal in Florida, and then the court decided to uphold the ban there, does that mean that any actions you took with your spouse (paying taxes, for example) were not in fact valid? Figuring this out in all of those states could be a mess, to say the least.

No matter what the court decides, it won’t change marriage across the country overnight. In that case of Utah Romero mentioned, as well as in many other states, putting a ban back into place could be a long process. Likewise, if the court finds that the bans are unconstitutional, same-sex couples in all states where there are currently bans, like Texas and Nebraska, can’t immediately run out and get married. That’s because this ruling technically will only extend to the four states where the cases the court is considering originated, Romero explains — Ohio, Kentucky, Tennessee, and Michigan. The ruling wouldn’t change things directly — or immediately — in other states.

“If the Supreme Court strikes down those states’ bans, that doesn’t immediately mean Georgia’s ban [for example] is ineffective,” he says. “There still needs to be court action to strike down Georgia’s ban, to apply the Supreme Court’s decision to Georgia. I would expect the federal courts to move quickly to do that. But all of that’s to say there’s going to be some delay.”