It is recommended that ladies wear smart attire with a hat or fascinator.

Source: https://www.dailymail.co.uk/femail/article-12222369/Royal-Ascot-racegoers-pull-stops-Ladies-Day.html?ns_mchannel=rss&ns_campaign=1490&ito=1490

Meanwhile, Zara Tindall, 41, was radiant in a blue tweed wraparound coat with a bow fascinator.

Source: https://www.dailymail.co.uk/news/article-11862719/Cheltenham-tragedy-eight-year-old-Malinello-falls-dies.html?ns_mchannel=rss&ns_campaign=1490&ito=1490

The practice has waned over time, but for the most part, people will still wear a hat or a fascinator to a black-tie event.

Source: https://torontolife.com/city/you-cannot-compete-with-the-crown-jewels-this-toronto-milliner-is-designing-hats-for-king-charless-coronation/

To accessorise at the 2018 wedding, the Duchess opted for a black Philip Treacy fascinator which worked perfectly with her dress.

Source: https://www.express.co.uk/life-style/style/1807646/meghan-markle-wedding-sunglasses-bra