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black tie event
noun phrase

- a formal occasion
- black tie: a dinner jacket/tuxedo and proper accessories

(Collins Dictionary)

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When people are expected to wear a certain type of clothing, a dress code exists. Dress codes for both men and women have changed with the times.

In the late 1860s, gentlemen belonging to the upper class in England began wearing dinner jackets instead of tailcoats to events beginning after 6 pm. Instead of white ties, the most elegant version of neckwear, they chose black ties.

By the turn of the century, "black tie" had come to refer to men's formal clothing, primarily to the dinner jacket. In US English, this type of men's suit is called a "tuxedo". It has satin or silk on the lapels and along the outer legs of the trousers.

The suit is usually accompanied by a waistcoat (vest in US English), or a cummerbund, a type of sash worn around the waist, along with a white shirt and a black bow tie. Women who are invited to a black tie event generally choose floor-length evening dresses or gowns.

Occasions that require formal clothing include dinners on cruise ships, balls, weddings, certain business celebrations, or opera performances.

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SYNONYMS
formal occasion, social function, gala

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SMUGGLE OWAD INTO A CONVERSATION TODAY
say something like:

"Our chairman is inviting all senior managers, together with their wives, to a black tie event to celebrate last year's sale's results."