Why Does Your Invite Say Costume de rigueur?

If you're invited to a ball or other social function and the invitation includes the French phrase costume de rigueur, you are expected to adhere to a very strict dress code—typically, a white tie and tails if you're a man and a floor-length evening gown if you're a woman. In French, de rigueur means "out of strictness" or "according to strict etiquette"; one definition of our word rigor, to which rigueur is related, is "the quality of being strict, unyielding, or inflexible." In English, we tend to use de rigueur to describe a fashion or custom that is so commonplace within a context that it seems a prescribed, mandatory part of it.

De Rigueur: How to Pronounce It, Spell It, and Use It in a Sentence

If you want to use de rigueur in conversation pronouncing it correctly is de rigueur. (Click here to find out how.)

Spelling this fancy French borrowing correctly, on the other hand, isn't de rigueur (your spellcheck will do it for you in most cases), but it is possible. The vowels of its final syllable are trickiest. It may help to remember other French borrowings that end in eur, such as amateur, chauffeur, and entrepreneur. And of course the last four letters of liqueur match de rigueur perfectly.

De rigueur has been used as an adjective in English for almost two centuries now, which means that it's established enough to appear in running text without italics. It's foreign-sounding enough, though, that people can feel tentative about using it. Apply it where synonyms like proper, correct, and decorous are at home. Here are some examples of it in use in its adopted language:

Anglophone parents worry that being too strict will break their kids' creative spirits. A visiting American mother was shocked when she saw a playpen in our apartment in Paris. Apparently, back home, even playpens are now seen as too confining. (We didn't know. In Paris they're de rigueur.)
— Pamela Druckerman, Bringing Up Bébé, 2012

Being in the business of writing about cocktails and bars, I often find myself in some pretty swank digs—various "mixology" dens where the elaborate drinks require complex techniques, house-made bitters and farm-to-table infusions are de rigueur, and the bartender has achieved celebrity-chef star status.
— Jason Rowan, Wine Enthusiast, April 2014

Although de rigueur is usually found after the verb (especially after is or are), it's also sometimes used in the traditional adjectival territory before a noun:

Stone, who patiently smiled through the de rigueur photo shoot in front of a backdrop emblazoned with the logos of the festival and its sponsors …
— Paul Liberatore, Marin Independent Journal (marinij.com), 6 Oct. 2016