Royal Baby: The term 'Heir and Spare' explained
The birth of Kate and William's new baby girl will mean a big adjustment for Prince George, who will have to deal with sharing some of the limelight with his younger sister.
While George is the "heir," this new princess will be known as the "spare," which is a role that some of her predecessors have struggled to define.
As heir to the throne, Prince George will be left in no doubt as to what is expected of him, but his younger sibling will have trickier shoes to fill.
The Queen’s younger sister, Princess Margaret, was a classic example of a younger royal sibling who often struggled with her role.
Constrained by her royal title, she suffered from boredom, often relieved by smoking and drinking.
She was unable to marry her first love, Group Captain Peter Townsend, because he was divorced, and even when she did get married, to the Earl of Snowdon, she continued to be surrounded by scandal and speculation.
Controversy has also dogged Prince Andrew, the Queen’s second son and “spare” to Prince Charles.
After leaving the Navy in 2001, he was appointed UK trade envoy, but had to resign over his friendship with child sex offender Jeffrey Epstein.
In his younger years, the new baby's uncle, Prince Harry, gained a reputation as the “Party Prince," hitting the headlines for a Nazi uniform gaffe, an infamous strip billiards session in Las Vegas and numerous visits to nightclubs.
But Prince William's "spare" has undoubtedly matured, largely thanks to his two tours of Afghanistan.
He is now planning a post-military career helping wounded service personnel.
Prince Harry may one day prove to be the perfect mentor for his new niece or nephew.
The arrival of a girl has also restored the gender balance amid a "dearth of young princesses" in the direct line of succession, a historian has said.
A girl has not been born this high up the line of succession for 65 years since Princess Anne arrived third in line in 1950.
Dr Judith Rowbotham, visiting research fellow at Plymouth University, said it was the women in the royal family like the Queen who have paved the way for the monarchy in recent history.
The historian said: "It's nice to have the gender balance restored. It has been unusual that there's been so little in the way of girls in the royal family for such a long time.
"George III had lots of daughters and so did Edward VII. There has been a dearth of young princesses in the close royal family."
She added: "Quite a number of queen consorts have been tough and formidable in their own right. What is nice is that women born into the royal family make their own way.
"It's the women in the last couple of centuries who have lived longer and in a sense done more and wider things.
"I think it's a good thing Prince George has a sister. It will take some of the pressure off him when he's older."
Royal author and historian Hugo Vickers said the new baby may one day be known as the princess royal.
Customarily given by the sovereign to his or her eldest daughter, the honorary style is currently used by Princess Anne.
"Everybody is very excited a princess has come along. There has not been a princess in direct line to the throne since Princess Anne was born in 1950," Mr Vickers said. "She will one day be entitled to the title princess royal."
The birth comes as the Queen prepares to overtake her ancestor Queen Victoria as the longest reigning British monarch in history this September.
"What is so wonderful is that the Queen is older than Queen Victoria and she's still firing on all cylinders. In the midst of the hot air of the General Election, this is a just a lovely thing," Mr Vickers said.