Derby hats are also commonly called bowler hats in the United States. The terms "derby hat" and "bowler hat" are often used interchangeably, though "bowler" is more common in Europe, while "derby" is favored in the U.S. Other less common names include billycock, coke hat, and bombín depending on the region

. The derby/bowler hat is a hard felt hat with a rounded crown and a curved brim. It was originally designed in the mid-1800s as a protective hat for gamekeepers and polo players, offering a close-fitting, low-crowned style that protected the head from low-hanging branches

. In contrast, the Kentucky Derby event features a different style of headwear known as fascinators , which are decorative headpieces often adorned with feathers, beads, and flowers and attached via combs or headbands rather than sitting loosely like traditional hats

Summary:

• Derby hats = Bowler hats (same style, different regional names)
• Hard felt, rounded crown, curved brim
• Originated in 1849 for protection during horseback activities
• Kentucky Derby "hats" are often fascinators, which are decorative headpieces, not traditional derby hats

This distinction clarifies that the classic derby hat is a type of bowler hat, while the fancy hats commonly seen at the Kentucky Derby are often fascinators