We notice AM and PM on our clocks and wall clocks every second of our time. In fact our whole life revolves around these two abbreviations. Let us understand what these two abbreviations stand for and what is their historical background.

AM is a short form of the Latin phrase ante merīdiem meaning “before midday”; PM is a short form of post merīdiem, meaning—“after midday.” The 24 hours of the day are divided into two periods, i.e. AM which means before midday and PM means after midday and each period comprises of 12 hours in number.

Historical Background

Clocks are one of the most primitive inventions of mankind. The term “clock” is derived via Dutch, Northern French and Medieval Latin from the Celtic term clagan and clocca which means “bell”. This device was designed to know the natural day/night division of a calendar day forming it into two cycles—to track the position of the Sun in day and to tracked the position of the Moon and stars in night.

It was Egyptians who divided the day into 24 equal parts by using the base of 12. The entire episode of clock and dividing time period was to reflect the rotation of the earth. That is why in ancient times sundial was used to measure solar time on light of the knowledge of latitude which was created in Babylon over six thousand years ago.

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Ambiguity behind 12 pm or Noon in 12 Hours clock

Since, AM stands for before midday and PM stands for post midday, hence, these terms cannot be used with 12 o’clock. In order to remove this hurdle, it was recommended that 12 PM should be denoted as 12 noon and 12 AM should be denoted as 12 midnight. That is why, each hour during the day, except midnight and noon, is suffixed by AM or PM to identify whether the hour being referred to is in the morning, afternoon or evening.

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