A COTTER pin is a type of pin used to secure two or more parts together. It is usually made of metal and has a head at one end and a split tail at the other. A HAIR pin is a type of pin used to hold HAIR in place. It is usually made of metal or plastic and has a curved shape. A linch pin is a type of pin used to secure a wheel or axle to a shaft. It is usually made of metal and has a looped head and a pointed tail. All three of these pins are types of pins that can be classified as 'kind of pin'.