After the death of Nelson Mandela, President Obama ordered that U.S. flags on government buildings be flown at half-staff until Monday evening — a symbolic gesture of a nation in mourning.
It's a tradition observed by countries around the world, one that began as early as the 17th century. Mental Floss reports:
"The oldest commonly accepted reference to a half-staff flag dates back to 1612, when the captain of the British ship Heart's Ease died on a journey to Canada. When the ship returned to London, it was flying its flag at half-mast to honor the departed captain."
Some scholars say this allows the "invisible flag of Death" to fly at the top of the pole.
In the U.S., President Dwight Eisenhower codified a set of rules in 1954 for when the flag must be flown at half-staff. (And yes, the U.S. calls it "half-staff," not "half-mast." You can use the term "half-mast" if you're in Canada or other locales or if you're in mourning on a boat.) They're largely formulaic: The death of a president merits 30 days; a vice-president, 10; a congressman, one to two.
The rules also allow the president to decide other periods of mourning when the half-staff is warranted. Most recently, Obama ordered the lowering of the flag to honor the victims of the Washington Navy Yard shooting and for the 50th anniversary of President John F. Kennedy's assassination.
But it's uncommon for the U.S. flag to be lowered in honor of foreign leaders' deaths. Obama issued a statement of mourning earlier this year after the death of Margaret Thatcher, former prime minister of the United Kingdom, but he didn't order the lowering of the American flag. In fact, the last foreign dignitary to be memorialized with the flag was Pope John Paul II in 2005.
Flags are also flying at half-mast in Mandela's honor in France, Canada, Norway, New Zealand, Bermuda, Kenya and, of course, South Africa.