When Congress issues a subpoena for documents, usually they get what they want. But that’s not the case for certain documents related to the 2020 Census question about citizenship, the Department of Justice said on Wednesday in a letter to Elijah E. Cummings, chairman of the House Oversight and Reform Committee. The reason: President Trump is invoking executive privilege. 

The committee voted to recommend holding Attorney General William P. Barr and Commerce Secretary Wilbur Ross in contempt of Congress for failing to comply with the subpoenas — though the committee’s Republicans say that the administration has already provided thousands of pages of relevant documents — but Trump is unlikely to face consequences for failing to comply. That’s because the doctrine of executive privilege, which Trump invoked for the first time in his presidency just a month ago, to block House Judiciary Committee Democrats from seeing Special Counsel Robert Mueller’s un-redacted report, gives presidents wide leeway to keep secrets.

So what is executive privilege? TIME once summed up the evolution of the concept as: “George Washington invoked it, Dwight Eisenhower named it and Richard Nixon abused it.” Here’s what else to know about the term and how its use has evolved over time. 

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The term “executive privilege” is not in the U.S. Constitution, but it’s considered an implied power based on the separation of powers laid out in Article II, which is meant to make sure one branch of government doesn’t become all-powerful; executive privilege is one way the legislative branch’s power over the executive is limited. For example, when Congress investigated George W. Bush’s firing of eight U.S. Attorneys in 2006, the White House Counsel at the time, Fred Fielding, alluded to executive privilege in a letterreferencing the “the constitutional prerogatives of the presidency.” But prominent constitutional law expert Raoul Berger famously called it a “constitutional myth” in his 1974 book, literally entitled Executive Privilege: A Constitutional Myth.