Each head has its own size, each it own particular shape. A hat may fit property, but if its form, width, height and breadth do not harmonize with the shape of the customer’s skull and face, a discordant note seems to spoil the appearance of the wearer. Head size is based upon the circumference of the head. If a string is placed around a head and it measures 21.875 inches and this circumference of the odd oval is made into a circle, the diameter of the circle would be 7 inches, the equivalent of a size 7. International hat sizes differ slightly. English sizes are based on a true diameter of a circle, while American sizes have a slight deviation. There is a 1/8 difference allowed in some hat charts.