With the news this week that Prince William and his wife Kate are expecting a second child, not only is there now a one-year-old heir to the throne, there will soon also be a "spare."

"Spare to the heir" may not be the most elegant moniker, but the unofficial title is nevertheless an important one for the Royal Family.

Under the rules of succession in the Commonwealth, when it comes to taking over the throne, preference is always given to the eldest child of a ruling monarch. Next comes his or her children, and then his or her children’s children.

(It was once tradition that British male heirs were favoured over females. In 2013, Britain and 15 other Commonwealth realms agreed to change succession laws to give female and male heirs equal status. The changes haven't yet taken effect, as they must be approved by all 16 nations first.)

Because the line of succession flows through the oldest child, royal children born as "spares to the heir" rarely have to worry about ascending to the throne.

Princess Anne, for example, the younger sister of next-in-line to the throne Prince Charles, was once second in line. Now, she ranks 11th.

Her younger brother, Prince Andrew, the Duke of York, was dubbed a welcome male "spare to the heir" when he was born in 1960. But Charles has had two sons since Andrew, as well as one grandchild. And now that another grandchild is on the way, when that child is born, Andrew will be bumped to sixth in line.

With a low chance of ascension, the "spare to the heir" has long tended to be the wilder child of a royal family.

Prince Andrew has often been criticized over the years for leading a wild-spending life, not to mention his marrying a woman who many have argued never quite fit the royal mould: Sarah Ferguson.

And of course, there's Prince Harry, younger brother to heir apparent Prince William. He has always been the more mischievous of the two, caught in several high-profile partying incidents and scandals in recent years, including scuffles with paparazzi and half-naked pool playing in Las Vegas.

Perhaps "spares to the heir" let down their guard in the public eye because they know they likely don't have to worry about ascending to the throne. But there have been a number of instances when royals not even considered as “spares to the heir” were suddenly forced to take over duties they hadn't expected.

Take, for example, Queen Elizabeth.

Her own father, King George VI, born Albert Frederick Arthur George, never expected to be king and spent most of his life in the shadow of his elder brother, Edward. But when their father, King George V, died in January 1936, Edward became King Edward VIII and Albert became heir apparent, as Edward was still unmarried with no children of his own.

Edward then set off a royal crisis when he decided to abdicate later that same year, in order to marry American socialite Wallis Simpson, forcing Albert to take the crown as King George VI and making Elizabeth the future queen.

In Monaco today, Prince Rainier III's eldest child, Princess Caroline, is currently the heir presumptive to the throne. That's because her younger brother, Prince Albert II, now the reigning monarch, still has no children. But that may change later this year; it was announced in May that Albert's wife, Charlene, is pregnant with a child expected by year's end.

If the baby is a girl, it will be remarkable for another reason: she would be the first female offspring of a still-serving monarch born in direct succession on the male line since 1897.

Current line of succession to the British throne:

Queen Elizabeth II (born 1926-present)

  1. Prince Charles, Prince of Wales (born 1948)
  2. Prince William, Duke of Cambridge (born 1982)
  3. Prince George of Cambridge (born 2013)
  4. Baby-to-be, to be born in 2015
  5. Prince Henry of Wales (known by most as Prince Harry) (born 1984)
  6. Prince Andrew, Duke of York (born 1960)