When men decided to divide the day into twenty-four hours, they used numbers one through twelve two times. As a result, there was one o’clock during the day and another one o’clock after midnight. This created confusion. If one was told to submit a project at six o’clock, did this mean six o’clock in the morning or at night?

The Romans provided a solution to this problem. They thought that noon time, the time when the sun is at its apex, is an important time. They called noon Meridies and measured time by this. They called the morning ante meridiem, which means “before noon” while “after noon” was called post meridiem. Ante meridiem was shortened to A.M. while post meridiem was shortened to P.M.