Cotter pins insert into holes of certain types of fastener systems to secure them into place. The bulbous end prevents the pin from being pulled through the hole so it is important to select a pin that has a diameter measurement larger than the hole diameter in the fastener you are working with. Once inserted through the hole, the two prongs at the opposite end can be separated and bent outward to lock the pin. Cotter pins are most frequently used to secure clevis pins or castle nuts.