- The Duchess of Cambridge is expecting her second child, Buckingham Palace announced
- "Throughout history, the only way to secure a throne has been with a phalanx of children"
- Aristocratic wives were once told they needed to provide an "heir and a spare" at least
- The second child has a vital role in carrying out royal duties and obligations
The Duchess of Cambridge's announcement today puts an end to months of pregnancy speculation. Number Two is on the way and with him or her comes a huge boost for the Royal Family. The level of excitement won't quite be the same as it was for Prince George -- but we can still expect great interest across the world and massive baby fever early next year.
But why does the "spare heir" matter?
One child is never enough for a monarch. Throughout history, the only way to secure a throne has been with a phalanx of children -- nine for Victoria, thirteen for George III.
Aristocratic wives were once told they needed to provide an "heir and a spare" at least -- and the same has always been true for the royals.
Once, this was because high mortality rates meant that the more children the better -- for the first born son didn't always come to be King. Henry VIII was the second son, who became heir and ascended at 17 after the untimely death of his brother, Arthur.
George V was also the second son -- and also became heir after his elder brother died young from influenza. Both men married the women who'd been betrothed to their brothers -- Henry made Catherine of Aragon his queen and George wed Mary of Teck.
The current Queen's father, George VI, was the second son of George V, and was never meant to be king. But when his elder brother, Edward VIII, gave up the throne in 1936 to marry American divorcee Wallace Simpson, the heir became the king -- much to his own surprise.
Now we don't expect the second child will come to rule, but he or she still has a vital role in carrying out royal duties and obligations. The Royal Family - or "Firm," as the current royals like to call it -- needs plenty of employees for the huge number of royal visits and receptions.
What is different in 2014 is that the heir and the spare heir can be female. After the law changed last year to allow women the same rights of accession to the throne as men, females are no longer pushed to the back of the line.
If the Duchess of Cambridge's baby is a girl, she will be the next in line to the throne after her brother -- even if more sons follow.
This baby will be a huge boost to the Royal Family - and "Brand Windsor" across the world. But what will his or her life be like? The role of the second-in-line is not easy.
For Princess Margaret being younger sister of the heir was no picnic -- she had all of the downsides of being royal, such as lack of privacy and restrictions on freedom, without the great compensation of becoming Queen. Unable to find a role for herself or marry the man she loved, the Princess sank into depression and ill health.
Prince Harry, too, has been clear on how hard it is to find an occupation -- he was eager to fight on the frontline in Afghanistan but was accidentally exposed by Australian media and had to return. It's also been hard for him to find a woman who will take on the royal role.
"Royals are only private in the womb," said the governess of Elizabeth II. We are fascinated by royal children -- most of all by the second or the "spare heir." What will be even more difficult for this second child is that he or she will grow up in a world of camera phones and social media, where private photos are gold dust and royals are the biggest celebrity big game of all.
The life of the second child can be exciting and dynamic -- but most of all it is one of extreme fame -- and a lot of responsibility.