President Donald Trump’s readiness to accept a luxury jet as a gift from the ruling family of Qatar for conversion into a presidential aircraft has revived the conversations around emoluments and the notion of a president otherwise allegedly profiting off the office.

“I would never be one to turn down that kind of an offer,” he told reporters Monday, after being asked if Qatar was getting anything in return for the plane. “I could be a stupid person and say, ‘No, we don’t want a free, very expensive airplane.’”

But there are constitutional prohibitions against the president receiving gifts from foreign entities or even domestic ones. It’s a conversation over emoluments, territory that Trump has been forced to navigate, and litigate, in the past.

What is an emolument?

Simply, an emolument is compensation for services, from employment or holding office, that can take the form of a salary, fee or profit.

What is the Emoluments Clause?

There are separate emoluments delineations in the U.S. Constitution. Both are aimed at preserving the independence of the president from influence from outside entities, including Congress, states and foreign governments.

Article I bars anyone holding government office from accepting any present, emolument, office or title from any “King, Prince, or foreign State,” without congressional consent.

Article II deals with domestic emoluments, noting that Congress can’t increase or decrease the president’s compensation during his term in office, and prohibits the president from receiving any emolument from the states.

Why is it coming up now?

Trump has reportedly been offered a Boeing 747-8 by Qatar in an arrangement that could be formalized as he travels to the Middle East this week. The Qatari government has said a final decision hasn’t been made. But Trump has defended the idea as a fiscally smart move for the country, even as critics argue it would amount to a president accepting an astonishingly valuable gift from a foreign government.

“If we can get a 747 as a contribution to our Defense Department to use during a couple of years while they’re building the other ones, I think that was a very nice gesture,” Trump said Monday at the White House.

The luxury 747 — currently parked at Palm Beach International Airport, close to Trump’s private Mar-a-Lago resort — would be donated to a future presidential library. Trump has said that he wouldn’t use it for personal travel after leaving office, suggesting that it would be decommissioned like the Boeing 707 that Ronald Reagan flew on in the 1980s, and which is on display at Reagan’s presidential library in Simi Valley.