A hat is a headdress worn for a variety of reasons, including protecting yourself from elements, ceremonial reasons (such as university graduation, religious reasons, security), or as a fashion accessory. In the past, hats were an indicator of social status. In the military, the hat can indicate nationality, service department, rank and / or regiment. The police usually wear distinctive hats, such as pointed hats or wide brimmed hats, such as those worn by the Royal Canadian Mounted Police. Some hats have a protective function. For example, helmets can protect the heads of construction workers from falling, British police custody helmets can protect the heads of officials, sun visors can block the face and shoulders from the sun, cowboy hats can prevent the sun and rain, Ushanka straw hat with folding ear flaps can keep the head and ears warm. Some hats are worn for ceremonial purposes, such as wearing (or carrying) stucco boards at University graduations. Some members of the profession wear hats, such as the chef's torque. Some hats have religious functions, such as gloves worn by bishops and headscarves worn by Sikhs.

The size of the hat is passed about 1 cm (1? 2 inches) to determine the head circumference. Depending on the manufacturer, inches or centimeters may be used. Felt hats can be stretched to fit their own style. Some hats (such as hard hats and baseball caps) are adjustable. Cheap hats have "standard size" hats, such as small, medium, large and extra large: the mapping from manufacturer to manufacturer, as well as style and style, measurement size to various "standard size" is different, which can be seen by studying various catalogs, such as Hammacher Schlemmer.