Why are US flags being flown at half-staff on Inauguration Day?

President-elect Donald Trump has expressed frustration that flags will be flying at half-staff when he takes office later this month.

It's an action put in place by President Joe Biden to honor the late President Jimmy Carter, who died last month at 100. It's not a timeline that Trump can do anything about — until after he takes office — although a large U.S. flag at Trump's home in Florida has been raised to full height following Carter's burial.

Here's what to know about why flags are lowered when a president dies, who can issue that order and how long the process lasts:

Why are U.S. flags being flown at half-staff?

On Dec. 29, Biden ordered that U.S. flags be flown at half-staff in honor of the late former president. It's an honor that indicates that the country or a state is in mourning.

The U.S. flag code lays out parameters for lowering the U.S. flag to half-staff, including a 30-day period for current or former presidents to cover flags at federal government buildings and their grounds, as well as at U.S. embassies and other facilities abroad, including military installations and vessels.

Flags can be lowered to commemorate the deaths of other officials, including the vice president, Supreme Court justices and members of Congress, although those periods aren't as long.

Flags can also be ordered lowered in other circumstances, including a national tragedy or on Memorial Day.

Since U.S. flag code states that no flag should fly higher than the American flag on the same pole or nearby, state flags get lowered during those periods, too.

How long will flags be lowered?

According to Biden's proclamation, U.S. flags will be lowered for 30 days from Carter's death, until Jan. 28.

With the inauguration on Jan. 20, that means that flags will be at half-staff when Trump takes office and for the first week of his administration.

Who decides when to lower flags?

According to the U.S. General Services Administration, the president, a governor and the mayor of the District of Columbia can order U.S. flags to be flown at half-staff.

What has Trump said about flags being lowered?

On Jan. 3, Trump posted on social media that ''Democrats are all ‘giddy''' about the notion that flags will be lowered when he takes office as president.

''Nobody wants to see this,'' Trump wrote. He added that ''no American can be happy about it. Let's see how it plays out. MAKE AMERICA GREAT AGAIN!''

Asked about Trump's post at a briefing earlier this month, White House press secretary Karine Jean-Pierre said Biden would not consider reversing or reevaluating the half-staff plans.

Can Trump decide to raise the flags?

Yes. U.S. flag code dictates that flags remain lowered for the 30-day period from the death of a former president. But that code isn't mandatory, so once he becomes president, Trump could technically override it.

That's what happened in February 1973, when then-President Richard Nixon opted to raise flags — which he had ordered lowered in mourning following the death of former President Lyndon Johnson — before the 30-day mark to honor the first American prisoners of war released from Vietnam. The hiatus only lasted a day, and flags went back to half-staff thereafter for eight days.

Flags had also been at half-staff when Nixon was sworn in for his second term in January 1973, due to Nixon having ordered them lowered after the death of former President Harry S. Truman.

There is a place where Trump can change flags ahead of taking office: at his Florida home. A large U.S. flag at Trump's private Mar-a-Lago club was initially lowered to half-staff according to protocol — and per directives by both Biden and Florida Gov. Ron DeSantis — but is now back to flying at full height, having been raised in the days after Carter was buried Thursday in his hometown of Plains, Georgia.

A spokesperson for Trump on Monday did not immediately respond to an emailed request for comment. Trump paid respects at Carter's casket as it lay in state in the Capitol Rotunda and also attended Carter's state funeral service in Washington last week.

Has Trump taken issue with lowering flags before?

Yes. After the death of Arizona Republican Sen. John McCain — with whom Trump had a fractious relationship — in 2018, the Stars and Stripes were briefly lowered to half-staff over the weekend but went back up to full height the following Monday, while flags at the U.S. Capitol and elsewhere stayed at half-staff.

The flag was lowered again to half-staff after complaints from people in both the Republican and Democratic parties. Former presidents spoke at McCain's funeral, but the senator's family made clear they did not want Trump to attend.

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Darlene Superville in West Palm Beach, Florida, contributed.

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Kinnard reported from Chapin, South Carolina, and can be reached at http://twitter.com/MegKinnardAP