In 1860, the Brooklyn Excelsiors wore the ancestor of the modern, rounded-top baseball cap, and by 1900, the "Brooklyn style" cap became popular. During the 1940s, latex rubber became the stiffening material inside the hat and the modern baseball cap was born. The "bill" or "brim" was designed to protect a player's eyes from the sun. Typically, the brim was much shorter in the earlier days of the baseball hat. Also, the hat has become more structured, versus the overall "floppy" cap of the 19th and early 20th centuries. The baseball cap was and still is an important means to identify a team by. Often the logo, mascot, or team's initial was placed on the cap. Usually, the cap was also fashioned in the official colors of a particular team.

The basic shape, including curved bill, is similar to some styles of 19th century sun bonnets ^[1] .