The character 壬 is formed from an ancient pictograph of a loom shuttle, evolved into its modern stylized shape and used as the ninth Heavenly Stem. The character 戌 originates from a pictograph of a weapon such as an axe, streamlined over time to become the eleventh Earthly Branch. Together, they combine as a fixed pair in the sexagenary cycle, where a Heavenly Stem and an Earthly Branch are joined to designate a particular year.