This map shows the world's remaining monarchies and how they treat gender in succession. Click to enlarge. (Max Fisher/The Washington Post)

Sometime in the early sixth century, the king of a small Frankish tribe called the Salii, who lived in parts of present-day Belgium and the Netherlands, asked a handful of local elders to put together laws for their society. What became known as the Salic Laws formally codified such traditions as inheritance, criminal punishment, and royal succession: Only a male heir, according to the laws, could take the throne.

The king who had commissioned them, Clovis the First, went on to conquer much of modern-day France and Germany. His Salic Laws were passed down, first orally and then in Latin, and gradually became the basis for much of Europe's legal practices. That included the requirement of male royal succession, which endured for centuries in Europe, including in the United Kingdom, which began the process of repealing it last year.

The news this week that the duke and duchess of Cambridge, Prince William and Kate Middleton, are expecting a child is drawing renewed attention to the U.K.'s effort, begun only last year, to repeal its law privileging men over women in royal succession. All 16 states in the British Commonwealth have agreed to repeal the requirement, which would mean that William and Kate's first child would be in line to the throne, regardless of gender. Still, though, the country's legislatures must ratify the change, including in the U.K. itself, where 18th and 17th-century constitutional documents must be amended. "Notwithstanding a few parliamentary turns of the wheel, this is now going to happen," U.K. Deputy Prime Minister Nick Clegg pledged.

But how typical is gender-equal succession in the world's remaining monarchies? Though the royal families governed by the Salic Laws have mostly been abolished – and the U.K.'s actually diverged from those laws long ago, before returning to a similar requirement around 300 years ago – the male-only requirement is still surprisingly common.

Above is a map – click it to enlarge – of the status of female succession laws in the monarchies of the world. The dark blue countries enjoy full equality, and they're all in Europe: Sweden was first to knock down the Salic tradition, passing a law in 1980 to allow equal succession. The Netherlands followed in 1983, Belgium in 1981, and Denmark in 2009. The light blue countries, members of the British Commonwealth, are ruled by the English monarch.

The yellow countries are interesting cases: countries, such as the U.K., considering laws to allow equal succession. Norway actually technically allows it already, having passed such a law in 1990, but it also granted an exception so that the current prince will take the throne instead of his older sister, which is why I've colored it yellow. Spain, Japan and Lesotho (the little country surrounded by South Africa) are all considering passing such laws, but not necessarily for reasons of gender equality. In Spain, intra-royal family politics are becoming a national issue; Japan and Lesotho both lack immediate male heirs, but the royal families are encountering severe public and political resistance against suggestions that they might pass the throne to daughters. It's fascinating to see these three countries, which could not be more different, all having a very similar version of the same debate: tradition versus progress.

The red countries do not allow women to take the throne under any circumstances: Morocco; the Middle Eastern states of Saudi Arabia, Jordan, Oman, Kuwait, Qatar, and Bahrain; plus, far away in Southeast Asia, Cambodia, which actually appoints its monarchs through a special council. The light-red countries – Thailand and Bhutan – give preference to male heirs but allow female heirs when no men are available to take the throne. Until very recently, the United Kingdom was among them.

Correction: This post incorrectly stated that the U.K.'s succession change had already been finalized. In fact, it is still ongoing.