WASHINGTON - When Pope Francis arrives in the United States, he will get an airport welcome that few world leaders have ever received: a plane-side greeting from President Barack Obama.

The extraordinary gesture on Sept. 22 is just the beginning of the pomp and protocol that Washington will put on display to welcome the popular leader of the world's 1.2 billion Roman Catholics and the head of Vatican City on his first U.S. visit.

The next day, Francis will be just the third pope to visit the White House, being greeted as most heads of state are, with his car pulling slowly up the South Lawn's driveway to the spot where a red carpet will be rolled out and Obama and his wife, Michelle, will be waiting.

Thousands of invited guests, including many Catholics, will gather on the lawn to receive Francis.

"Like millions of Americans, I am very much looking forward to welcoming Pope Francis to the United States," Obama told a prayer breakfast this year.

The president and the pope met for the first time in March 2014 at the Vatican, and Obama has been outspoken about his admiration for the 78-year-old native of Argentina. Despite differences over abortion, the two have found common ground on such issues as U.S. policy toward Cuba and Iran, climate change, and poverty and income inequality.

Nothing that happens behind the iron gates of 1600 Pennsylvania Ave. on the late-September morning of Francis' visit will come as a surprise to the Holy Father. It is standard for White House staff, including from the first lady's and social secretary's offices, to work out the details of appearances like these beforehand with the visitor's representatives.

"So much of arrival ceremonies are steeped in history and tradition," said Anita McBride, who was an assistant to President George W. Bush, who received Pope Benedict XVI at the White House in April 2008. "Every arrival ceremony is important. Every single foreign visitor is treated with the same level of respect and planning."

Francis has shown little interest in protocol and the trappings of his office, shunning both the fancy papal apartment and motorcade, for example. But he does put himself through the motions of diplomatic etiquette to be respectful of his hosts.

The motions at the White House for Francis will last for just about 90 minutes, the sum total of the pontiff's visit.

Shortly after he arrives on the South Lawn, the anthems of the U.S. and the Vatican will play following military honors that include a 21-gun salute and band members dressed in colonial garb. Obama will welcome the pope with remarks followed by a reciprocal address from Francis. They will then head inside the White House and reappear on the balcony.

Inside, Francis will sign the official guest book. He and Obama will exchange gifts before their Oval Office meeting.

While a handshake is considered an appropriate way to greet Francis, Catholics have the option of kissing the gold ring the pope wears on his right hand, according to the State Department protocol office. Vice President Joe Biden, a devout Catholic, opted not to kiss the ring after he attended Francis' installation Mass at the Vatican in March 2013.

Dark-colored clothing is recommended for the occasion. Women's hemlines must cover knees; sleeves must hide elbows.

He should also be addressed as "Your Holiness."

Popes and U.S. presidents share a nearly 100-year history of interaction, with more than two dozen meetings since the first one at the Vatican on Jan. 4, 1919, between President Woodrow Wilson and Pope Benedict XV after World War I.