If we had this inheritance principle in Britain, Prince Charles would lose his pre-eminent right to succeed to the throne to his younger brother Andrew.
That’s because porphyrogeniture refers to a child who has been born to a reigning monarch and specifically to a right of succession to the throne based on having been so born. Queen Elizabeth’s two eldest children, Charles and Anne, were born before she became queen in 1952, but Andrew was born in 1960.
The prefix here, porphyro-, derives from classical Greek porphura for the colour purple. The second part is from Latin genitura, a person’s birth, which is from the root of gignere, to beget. So the word means “born to the purple”, an English idiom dating from the seventeenth century for being born to a reigning monarch. Porphyrogeniture was created around 1860 as a high-flown Latin version of the idiom.
In ancient times, to wear clothing coloured purple was the prerogative of royal or imperial families, because the dye, Tyrian purple, was rare and costly. (It was a variable shade often much nearer red than we would now consider purple; it was also called Tyrian red.) This was probably where the idea of being born to the purple came from, though it was reinforced by Byzantine empresses who gave birth in a room in the Constantinople palace that was lined with a purple stone called porphyry.
Porphyrogeniture isn’t as rare a word as you might think. Historians find it useful when discussing some cases of disputed royal succession, where having been born to a reigning monarch potentially trumped the rights of an older son who had been born before his parent became sovereign.
It’s a member of a set of words of which the best known is primogeniture, which is strictly speaking the state of being the first-born child, but which is more often used for the practice of making the first-born male child the automatic heir. Others are secundogeniture, the right of succession of a second son, the rare tertiogeniture for the third son and ultimogeniture for inheritance by the youngest son, which was once the custom in some counties in southern England.