The Chinese calendar is based on numeric cycles of 60. Originally it was used for cycles of days but from around the year 1OO B.C. it came to be used for years. Every year is given a unique name by the combination of two characters from two separate sets. The first is one of the Ten Heavenly Stems. The second is one of the Twelve Earthly Branches. Of the 120 possible combinations only 60 are used. The non-recurring combinations are known as the jiazi. To be able to identify which 60 year cycle the date is referred to, the cyclical year characters are often combined with the Imperial reign.