Even Donald Trump preempted the president, ordering flags on his own U.S. properties lowered on Tuesday.

Citing false rumors that Obama had lowered the flag when pop singer Whitney Houston died — in fact, it was New Jersey Gov. Chris Christie who did her the honor — retired pitcher Curt Schilling slammed the president on social media.

Flags at half mast for Whitney Houston? 4 Marines and 1 Navy serviceman assassinated by a terrorist on our soil…..nothing? — Curt Schilling (@gehrig38) July 20, 2015

Where the Charleston shooting had stirred soul-searching over the place of the Confederate flag in America, the Chattanooga massacre risks turning the Stars and Stripes into a political punching bag.

Partially obscured by the controversy is the reason why the flag is lowered in the first place.

The flag is to be flown at half-staff “when the whole nation is in mourning,” according to the Department of Veterans Affairs.

“By order of the President, the flag shall be flown at half-staff upon the death of principal figures of the United States Government and the Governor of a State, territory, or possession, as a mark of respect to their memory,” according to the United States Code. But he has the discretion to lower it as well “whenever he deems it to be appropriate or desirable.”

Recent presidents have “deemed it appropriate” for, among others, Pope John Paul II, Rosa Parks, Neil Armstrong, former House Speaker Thomas Foley and the victims of the Virginia Tech and Newtown shootings, the Indian Ocean tsunami and the Boston Marathon bombings. Half-staff flags were flown for foreigners Winston Churchill, Anwar Sadat, Yitzhak Rabin and Nelson Mandela, among others.

Among the earliest examples of a half-mast flag in the United States was in 1799, when the Navy Department ordered all Navy vessels to “wear their colours half mast high” on the occasion of the death of George Washington.

But why half-staff? Look to history.

“According to one line of scholarly thinking, by lowering the Union Jack, the sailors were making room for the invisible flag of Death,” reports Mental Floss’s Ethan Trex. “This explanation jibes with the British tradition of flying a ‘half-staff’ flag exactly one flag’s width lower than its normal position to underscore that Death’s flag is flapping above it.”

The practice dates back to at least 1612 and an ill-fated mission.