At 133 words long, Prince Philip’s full title includes the words duke, earl, baron, royal knight, and just about every other noble designation you can think of. But the highest and perhaps most obvious moniker is conspicuously missing: king.

If you’ve watched The Crown or spent a lot of time reading about British monarchs, you might be aware that women who marry kings are referred to as queens—the most recent example was Queen Elizabeth II’s mother, who was known as Queen Elizabeth after her husband assumed the throne. The technical phrase is queen consort, where consort basically means that you gained your title through marriage. Why, then, did Prince Philip not become King Philip (or king consort) after his wife’s coronation?

According to Reader’s Digest, although parliamentary law doesn’t consider gender in determining the line of succession, it does factor it into the designation of titles. In general, a wife assumes the female form of her husband’s title. This is why Meghan Markle became the Duchess of Sussex when Prince Harry was named the Duke of Sussex, and it’s also why Kate Middleton will likely become Queen Catherine if and when Prince William is crowned king.

Husbands, on the other hand, don’t automatically match their wives’ titles, especially when it comes to the reigning monarchs. As Town and Country explains, queen can still be a symbolic title, but king only ever describes a reigning monarch. In other words, the word queen can either mean “female monarch” or “the wife of a monarch,” whereas king can only mean “monarch.” It’s a vestige of a historically patriarchal system of government that used to value sons over daughters (and it also sheds light on why kings rank higher than queens in a deck of cards).