Ever since their wedding back in October, Princess Eugenie and Jack Brooksbank have been laying pretty low as a royal couple (save for their first official joint engagement in March). So we haven't really had a lot of time to dissect the protocol around Eugenie's new marital status (we've been sorta distracted by a little guy named Archie, too)—that is, 'til now.
Traditionally, as a married woman, Eugenie would become Eugenie Brooksbank (though this is becoming less common in favor of more modern name-taking approaches).
But this isn't any ol' plebeian couple. The granddaughter of Queen Elizabeth can't just (*gasp!*) take a commoner's name and add it to the end of her own. However, according to documents from the Court Circular (aka the official diary of the royal family), Eugenie has decided to honor her husband's lineage...just not in the way you'd expect.
According to the Court Circular's listing of the queen's garden party at Buckingham Palace last week, Eugenie's new full title is Princess Eugenie, Mrs. Jack Brooksbank. By using her new husband's name in this style, she's still following the tradition that royals in line for the throne don't use last names (she's currently tenth behind her older sister Beatrice), but also attaching herself to her spouse in a sweet gesture of solidarity.
The exception to the no-last-name rule is, of course, Master Archie, whose last name is listed on his birth certificate as Mountbatten-Windsor (a combination of both Prince Philip and Queen Elizabeth's formal surnames). This may be because Meghan Markle and Prince Harry want Archie to grow up as normal as possible instead of being referred to by first-name only like all the other working royals...though having that last name kinda defeats the purpose, no?
Whatever the case, Eugenie's got this whole "navigating royal protocol" thing down pat.