News that Catherine, the Duchess of Cambridge, is in the early stages of labor has set the Internet abuzz with curiosity. And in a historic first, Kate's child is guaranteed to be in the line of succession for the British throne — regardless of its sex.

Since days of yore, the royal line of succession to the British throne was — like in most monarchies — based on primogeniture, which traditionally gives preference to the firstborn male heir of a king and queen, meaning he inherits the title, lands and all other property belonging to his family.

One benefit of primogeniture is that it keeps all the lands belonging to a family intact and profitable: In many agricultural societies, divvying up land into smaller parcels that are too small to support an extended family would quickly reduce a family to penury. [The 10 Most Powerful Modern Women Leaders]

In the British royal family today, the throne will pass from Queen Elizabeth II upon her death to her son Prince Charles. Next in line is Charles' eldest son (and Kate's husband), William, the current Duke of Cambridge.

Because of a recent dramatic break with tradition, William and Kate's eldest child will be next in line to inherit the throne, regardless of whether the child is a boy or a girl.

The change was adopted in 2011, when the leaders of 16 British Commonwealth countries (including Australia, Canada, Jamaica, New Zealand and other countries) met in Perth, Australia, to review the traditional line of succession.

The 16 leaders voted unanimously to alter the centuries-old rule of succession to include daughters as well as sons. Previously, daughters could only inherit the British throne if there were no living sons.

Another age-old rule that was tossed out in 2011 stipulated that no heir could assume the throne if he or she were married to a Roman Catholic.

That rule was based upon centuries of religious persecution and warfare going back to the reign of Henry VIII, who broke with the Roman Catholic Church to divorce his wife, Queen Catherine, and marry Anne Boleyn.

But because one of the duties of the British monarch is to head the Church of England, even today, no Roman Catholic can hold the crown.

In a 1994 interview, Prince Charles caused a royal kerfuffle when he stated that he would rather be seen as a "defender of faiths," including Catholicism and other religions, according to CNN.

Another form of inheritance is known as ultimogeniture, in which the last-born child (again, usually a male) inherits the lands and titles of his parents.

Ultimogeniture is sometimes used to compensate the youngest son for remaining in the home longer and caring for his elderly parents. It was practiced in a handful of medieval European realms and in certain Japanese dynasties.

Strict primogeniture and ultimogeniture are relatively rare; in most cases, all male and female heirs are provided with some share or lands, titles or other rewards, even if they don't inherit a royal crown.

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