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What is A.M. and P.M.

A day has 24 hours. It is divided into two 12-hour periods. The first period is from 12 midnight to 12 noon. It is called Ante Meridiam that is before midday and is written as A.M.
The second period is from 12 noon to 12 midnight. It is called Post Meridiam that is, after midday and is written as P.M.

Noon and midnight actually divide the two periods and hence are written as 12 noon or 12 midnight. Thus, 10 o’clock in the morning is written as 100:00 A.M. whereas 10 o’clock at night is written as 10:00 P.M.
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