The "spare to the heir" is often a difficult role to fulfil within the Royal Family.
Second-born royals are characteristically less cautious and enjoy the freedom that comes with not having to prepare to rule as monarch.
But the position is open to criticism as the privileged and sometimes troublesome younger sibling to a future sovereign attempts to carve out a life for themselves amid the scrutiny of being an HRH.
Prince Harry, the Duke of York and the late Princess Margaret were all spares to the heirs and each encountered criticism over their conduct.
Now the Duke and Duchess of Cambridge's second child will be born as next in line after an older sibling and grow up in the same position.
Unlike Prince George, this Prince or Princess will be free from the responsibility of one day having to become monarch.
Whereas George is expected to eventually become king, as well as head of the armed forces and the Church of England, and possibly head of the Commonwealth, the younger Cambridge sibling is likely to have a less restrictive future.
Prince Harry has traditionally been dubbed a party prince in contrast to his older brother William - a future king. It is Harry who has scuffled with paparazzi photographers and been pictured naked playing strip billiards in Las Vegas.
As a child, Harry was always the more mischievous of the two princes.
Royal writer Christopher Warwick compared the brothers to the Queen and her younger sister Princess Margaret.
"William and Harry are the Elizabeth and Margaret of this generation. William is the dedicated, dutiful one who will perhaps one day be king. The other sibling, like Margaret, who didn't have these responsibilities and duties, can pursue a different type of private life," Mr Warwick said.
As a child, Princess Elizabeth was sensible and responsible, while Princess Margaret Rose was vivacious, naughty and fun.
Margaret, who died in 2002, was known for her glamorous lifestyle and turbulent love life. In the 1950s she fell in love with the divorced Group Captain Peter Townsend but renounced him after coming under political, family and church pressure.
She wed and later divorced photographer Antony Armstrong-Jones, who became Lord Snowdon, and went on to have an affair for several years in the 1970s with Roddy Llewellyn, who was 17 years younger.
The Duke of York, who until the arrival of William in 1982 had been next in line after the Prince of Wales, has also faced controversy.
He is nicknamed Air Miles Andy because of his jet-setting and frequent use of helicopters and his suitability as the UK special representative for trade and investment was criticised throughout his 10 years in the role.