Piles of books, an entire topiary, a block of Stilton cheese, a pink flamingo — all these and more have appeared on the heads of attendees at Royal Ascot, the five-day British racing event that is as famous for its outrageous millinery as its stallions. Hats may follow horses around the world, from the Kentucky Derby to the Dubai World Cup and the Hong Kong Cup, but nothing tops the … well, toppers at Ascot, founded in 1711 by Queen Anne. They go together like strawberries and cream. Pimm’s and cucumber. Champagne and finger sandwiches.

Perhaps it’s because of “My Fair Lady” and the extraordinary black and white lace hat Cecil Beaton created for Eliza Doolittle when she made her Ascot debut. Or maybe it’s thanks to Queen Elizabeth II, whose twin passions for horses and hats reached their apex at Ascot. At this point, though, the game of hat-upmanship is as heated, and closely watched, as the races themselves.

“To err on the side of extravagance as opposed to modesty is a joy for everyone,” said the milliner Stephen Jones, who reported that he once made an Ascot hat out of feathers for Jasmine Guinness, the aristo-model, in the shape of a feathered pinwheel about two feet high. “There are so few rules in dressing anymore that to dress up is part of the reason for going to Ascot. It’s the opportunity to do something you never do.”

Indeed, the Ascot dress code dictates that all attendees sport (ahem) hats — and in the Royal Enclosure, the most exclusive ticket area, and the Windsors’s spot, they must include a base that is at least four inches in diameter. That means “fascinators,” those bizarre concoctions of net and sparkle that sit on the edge of a headband like a bird on a twig, are not allowed. Instead, guests may wear a “hatinator” — a compromise between a fascinator and an actual hat — which may be why the default is often to go bigger rather than smaller.