Over the years they've seen it all - the births of both Prince George and Princess Charlotte, been there for Princess Diana's funeral and celebrated the Queen's 90 birthday. Some believe it was to verify that there was no switching at birth or other shenanigans.
Her Majesty The Queen was the first to be notified of the birth.
The choice of red makes sense, given her penchant for primary colours for the previous births and it looks fabulous with her flowing hair, styled to perfection as usual.
Meanwhile, British Prime Minister Theresa May on Monday congratulated Prince William and his wife Kate on the birth of their new boy.
Following the photo op, the royal couple will head back to Kensington Palace to begin their life as a family of five. The couple now live at Kensington Palace and past year William gave up his day job as an air ambulance pilot to concentrate full-time on royal duties on behalf of his grandmother, having faced some criticism from British newspapers who dubbed him "workshy Wills".More news: Philadelphia's top cop apologizes to black men arrested at Starbucks
"It is tradition that a framed notice of birth goes on display on a ceremonial easel on the forecourt at the Palace", a tweet explained. George in 2013 and Charlotte in 2015 were also born. Bookmakers had made Albert, Arthur, Jack and Fred the favourite names for a boy. The young royal's name is not expected to be announced until tomorrow.
William and Kate will mark their own seventh wedding anniversary on April 29.
Traditionally, a female royal would be bypassed by a younger brother, which is why Prince Andrew and Prince Edward and their respective children are closer to the throne than Princess Anne, who is older.
It is not known exactly what time the royal couple will appear with the new born.
William, who is destined to become king after his father Prince Charles, and Kate met at St. Andrews University in Scotland.