Here's why US flags are 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 honour the late President Jimmy Carter, who died Sunday at 100. It's not a timeline that Trump can do anything about until after he takes office.

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 US flags being flown at half-staff?  On Sunday, Biden ordered that US flags be flown at half-staff in honour of the late former president. It's an honor that indicates that the country or a state is in mourning.

The US flag code lays out parameters for lowering the US 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 US 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 US 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, US 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 US General Services Administration, the president, a governor and the mayor of the District of Columbia can order US flags to be flown at half-staff.

What has Trump said about flags being lowered?  On Friday, 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 Friday's briefing, 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. US 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 honour 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 due to the death of former President Harry S. Truman.

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 US 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.