A flat cap is a (typically) fabric cap with a modest rigid brim at the front of the cap. The most notable feature of a flat cap is the fact that the brim is actually incorporated into the construction of the cap itself, providing a smooth transition between brim and cap; as opposed to a baseball cap which is constructed out of two separate objects that are attached at a seam. Flat caps are constructed out of many different fabrics, though the most common are tweed and cotton. Leather is also a possible construction material, though its use is rare.
The flat cap can be dated back to 14th century Britain and Ireland though interestingly the style did not gain popularity on its own; instead it took an act of Parliament to firmly entrenched the flat cap in the public mind. As a means of stimulating wool consumption within England, Parliament made it mandatory for all males to wear a wool cap on Sundays and holidays. Because the flat cap was inexpensive, and conveniently made out of wool, it quickly became the go to hat of choice among most males.
Today the flat cap is still popular in Europe and North America. It is primarily worn by older individuals however there are certain groups within youth culture that have adopted the cap. It is also a popular fashion accessory among celebrities who are looking to add retro appeal to their wardrobe.