Prince George, Princess Charlotte and Prince Louis, the children of Prince William and Kate, Duchess of Cambridge, made their debut at Trooping the Color on Thursday riding in a carriage.

The siblings sat in a carriage alongside Kate and Camilla, Duchess of Cornwall.

Louis sat in the center flanked by his older siblings, George and Charlotte, as they waved to the crowds.

Trooping the Color, also known as the Queen’s Birthday Parade, is considered one of the most spectacular ceremonies in the world.

Each year on the reigning monarch’s official birthday, the streets around Buckingham Palace are lined with flag-waving crowds who gather to watch as 1,400 soldiers and 10 military bands parade alongside members of the royal family, who travel on horseback and in carriages.

The parade’s name comes from a military practice in which soldiers gather, or troop, around the Color, a flag that marks a regiment’s position in a battlefield.

At the end of the parade, Queen Elizabeth II traditionally carries out an inspection of the troops and receives a royal salute.

This year, for the first time in the queen’s 70-year reign, her eldest child, Prince Charles, will carry out that duty for her.

Elizabeth will arrive on the balcony of Buckingham Palace, where the royal family gathers to watch a military fly-over.

The queen has attended Trooping the Color every year of her reign except for one, in 1955, when it was canceled due to a national rail strike.

Prince Harry and his wife Meghan, the Duke and Duchess of Sussex, will join royal family members to watch Trooping the Color from a Buckingham Palace office that overlooks the parade ground, a spokesperson for the duke and duchess told ABC News.

The Sussexes look forward to watching from the Major General’s Office, the spokesperson said.

Senior working members of the royal family, including Harry’s brother, Prince William, and his wife Kate, along with their three children, will watch Trooping the Color, also called the Queen’s Birthday Parade, from the balcony of Buckingham Palace, alongside Queen Elizabeth II herself.

Because they are no longer senior working royals, Harry and Meghan are not part of the traditional balcony appearance, a decision made earlier this year by the queen, according to Buckingham Palace.

Queen Elizabeth II will miss part of Thursday's Trooping the Color, the kick-off to her Platinum Jubilee celebration, Buckingham Palace has announced.

The queen will not join other royal family members in the traditional carriage rides to the Horse Guards and will instead appear on the balcony of Buckingham Palace, the palace confirmed Wednesday.

Prince Charles, the heir to the throne, will take the royal salute on the parade ground and inspect the troops on the queen's behalf, joined by his son, Prince William, and his sister, Princess Anne.

The queen will take a salute from the cavalry as they go past Buckingham Palace on their return from the parade.

This is the first time in Elizabeth's 70-year reign that she will take a salute from the balcony at Buckingham Palace, and the first time Prince Charles will stand in for her on the parade ground.

Queen Elizabeth II shared a written thank you message on Sunday to mark the end of four days of celebrations for her Platinum Jubilee.

"When it comes to how to mark seventy years as your Queen, there is no guidebook to follow. It really is a first. But I have been humbled and deeply touched that so many people have taken to the streets to celebrate my Platinum Jubilee," Elizabeth wrote. "While I may not have attended every event in person, my heart has been with you all; and I remain committed to serving you to the best of my ability, supported by my family."

"I have been inspired by the kindness, joy and kinship that has been so evident in recent days, and I hope this renewed sense of togetherness will be felt for many years to come," the queen continued. "I thank you most sincerely for your good wishes and for the part you have all played in these happy celebrations."

The queen signed her message with her first name, Elizabeth, followed by the letter R, which stands for Regina, the Latin word for queen.