The cocked hat is best known for its use by naval officers prior to World War Two. Essentially a cocked hat is a wide brimmed hat that has had two parallel sides (of the brim) pinned together to form a hump in the middle of the hat that rises above the crown.
The cocked hat is a derivation of the bicorn hat which was most notably worn by Napoleon Bonaparte. The main difference between the two is the orientation in which the hat is worn. With a bicorn hat the points, or edges of the brim that are not pinned up in the middle, are aligned with the shoulders. Whereas a cocked hat is worn with the points running front to back.
Depending on the uniform or occasion it was popular to embellish cocked hats with everything from metal chains to feathers and medallions. Today cocked hats are rarely worn. Aside from their use in theater and costuming, cocked hats are a rare thing to see; though for a select number of highly formal military occasions cocked hats are occasionally used.