The Shang dynasty developed a calendar system. It was based on a period of sixty days. This was further divided into six, ten day 'weeks'.

The symbol for each day in this period is made up of two characters. The character on the top shows which day of the 'week' it is (day one, day two, day three....day ten). The character on the bottom is taken from a cycle of twelve characters.

They always follow the same order so that...

always comes before

and
always comes before

Once they have gone through this twelve day cycle they start with the first symbol again. When put together they make the sixty day cycle.