One measure of the success or failure of House Democrats’ impeachment inquiry will be just how much evidence they’re able to squeeze out of the Trump administration. There’s already a surprising amount of information out in the open about President Trump asking Ukraine to investigate Joe Biden’s son Hunter. But now House Democrats must figure out whether Trump’s actions are impeachable, which has meant pursuing information from a wide range of sources within the administration, from Vice President Mike Pence to heretofore unknown figures like European Union ambassador Gordon Sondland.

The White House is, to put it mildly, not on board. Earlier this month, White House lawyers sent the Democrats a fiery letter declaring that the executive branch would refuse to cooperate with the investigation in any way. Since then, other administration officials have also said they’ll refuse to cooperate — including some, like Defense Secretary Mike Esper and Energy Secretary Rick Perry, who have said the information requested by the House is confidential and may be covered by executive privilege.

We’re likely to hear more about executive privilege over the coming weeks as the tug-of-war between the Trump administration and Congress escalates. And that’s because if you are a president trying to withhold information from other branches of government, claiming executive privilege is an obvious place to start. It’s a powerful but sometimes murky concept that allows presidents to shield certain information about goings-on in the West Wing from the public, the courts, and Congress. But it’s also not a limitless power. Just ask Richard Nixon, whose presidency came to an abrupt end after the Supreme Court ruled that he couldn’t use executive privilege to avoid turning over a set of incriminating White House tapes.

So if you find the whole concept of executive privilege mysterious (and you are surely not alone), here’s a quick crash course on what it is and how it could affect the impeachment inquiry.

What is executive privilege? How does it work?

Presidents talk about a lot of sensitive issues within the confines of the White House — matters of national security, for instance, are a big one. And there are plenty of legitimate reasons why a president wouldn’t want the contents of those conversations to be broadcast outside the Oval Office — if advisers had to worry that their counsel would be made public, it would be very hard to have candid internal discussions. Enter executive privilege. It’s a power that allows the executive branch to ignore Congress’s and the courts’ requests for some documents or testimony that’s related to either presidential communications or the process of creating government policy.

Where does this power come from?

Executive privilege is not in the Constitution, but that hasn’t stopped pretty much every president from trying to keep information out of the hands of Congress. As a formal legal concept, though, it’s still relatively new. The term “executive privilege” was coined during the Eisenhower administration. It was first recognized by the Supreme Court as a legitimate presidential power in 1974 as part of the Watergate investigation — although ironically, the justices went on to conclude that even though executive privilege existed, it was not unlimited, so Nixon still couldn’t use it to avoid handing over the White House tapes to a court.

To be clear, there’s nothing inherently wrong with exercising a power that’s not explicitly spelled out in the Constitution. But executive privilege’s vague origins do mean that we tend to find out about its limitations only when a president tries to use it in a legally questionable way — which means there are many gray areas in how it can be used.

What have the courts said about executive privilege?

Surprisingly, not all that much. In the Nixon case, the justices were clear that executive privilege isn’t a literal get-out-of-jail free card, in that a president’s ability to use it to avoid handing damaging information over to a prosecutor is very limited. But they didn’t say much more than that, and haven’t ruled on it since.

Over the years, though, some limitations have been hashed out in a handful of lower court rulings and by the Department of Justice. For instance, experts told me there is broad agreement that only the president can invoke executive privilege, but he can still assert it to keep aides from testifying in some circumstances. However, the president can’t use it willy-nilly even if he’s dealing with Congress and not a criminal investigation. He generally has to have a good reason for withholding information, rather than simply refusing to share something because he doesn’t want Congress to have it. What that has meant in practice is that declaring an entire topic or category of people off-limits isn’t generally allowed.

How has Trump been using executive privilege?

So far, Trump hasn’t been extensively asserting executive privilege in response to House Democrats’ impeachment inquiry. But administration figures like Pence who have claimed that they don’t have to cooperate because the entire inquiry is constitutionally illegitimate have also noted that executive privilege could be invoked later. And in House Democrats’ other investigations, Trump has been aggressive about his use of executive privilege, going so far as to claim it for people like former campaign manager Corey Lewandowski, who never worked in his administration. So it seems likely that Trump will be continue to assert it going forward.

If Congress challenges Trump’s privilege claim, who decides who wins?

In the past, a presidential assertion of executive privilege was the opening gambit in a long negotiation that often — although not always — ended with compromise. But if the president doesn’t want to give an inch, the courts have to weigh in.

But there’s still an element of gamesmanship to these disputes, as the legal process can take a very, very long time to resolve. For instance, a battle over documents related to a botched arms-trafficking operation conducted under the Obama administration ping-ponged through the courts for seven years. Needless to say, House Democrats do not want to wait anywhere near that long to make decisions about impeachment.

Granted, if there were a fight over whether the Trump administration has to comply with subpoenas in an impeachment inquiry, it would almost certainly be expedited. But that wouldn’t stop White House lawyers from trying to slow-walk the process and hope that it kills Democrats’ momentum. And even if the Supreme Court decided to swoop in and short-circuit the legal process, it’s hard to imagine how the justices could weigh in by the end of November, when some Democrats have said they want to have articles of impeachment written up.

Who’d have the edge in a legal fight — the president or Congress?

If comes down to a legal fight, the Supreme Court would probably end up making the final call. And it wouldn’t be a slam dunk for either side. Several legal experts told me that in a previous era, they’d expect the justices to lean toward Congress, since Trump’s use of executive privilege is so aggressive. But the high court has never weighed in on the scope of executive privilege in a congressional investigation, and the current conservative majority might be a friendlier audience for the president, given some of the justices’ views on executive power. For instance, Justice Brett Kavanaugh suggested at one point in his career that the case involving the Nixon tapes might have been wrongly decided.

What if Democrats don’t have time to take Trump to court?

Democrats’ best bet, if they don’t want to trigger a lengthy legal battle, may be to get members of the administration to defy orders and testify anyway. And that already seems to have happened in at least one case. Lawyers for the Trump administration warned Fiona Hill, a former Russia analyst for Trump, that information about “diplomatic communications” would be covered by executive privilege. But Hill’s lawyers pushed back, contending (among other things) that executive privilege doesn’t apply when there’s been government misconduct. Hill did reportedly testify about diplomatic communications (her testimony was given at a closed-door hearing, so we don’t have all the details), but she hasn’t faced any consequences yet, so that may embolden others.

There’s also reason to think that if Trump’s use of executive privilege was particularly egregious, it could be spun as an impeachable offense itself. It wouldn’t be the first time — in the impeachment proceedings against Clinton, the president’s use of executive privilege was called frivolous and corrupt by House Republicans. So if Democrats can’t get all of the information they want, they could at least use Trump’s refusal to provide it as more fodder for impeachment.