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Posted by on Dec 1, 2015 in Tell Me Why |

What Do A.M. And P.M. Mean?

What Do A.M. And P.M. Mean?

“A.M.” and “P.M.” are the expressions we use to indicate before noon and after noon. They are abbreviations for “ante meridiem” and “post meridiem.” In timekeeping, the hours in a day are usually divided into two sets of 12 hours—from midnight till noon, and from noon until the next midnight.

The word “meridiem” comes from Latin and means “midday,” or noon. We call the first part of the day A.M. (ante meridiem, usually written “a.m.”), meaning “before noon,” and the second part P.M. (post meridiem, written “p.m.”), meaning “after noon.”

Each hour, except midnight, which is also denoted as 0:00, and noon, which is known as 12:00, are suffixed by AM or PM to identify whether the hour being referred to is in the morning, afternoon or evening. These two terms help keep ambiguity at bay in the 12-hour time system. There is a third, lesser-known abbreviation in this system: m. from the Latin merīdiēs meaning “midday” means noon.

However, m. is rarely used and might confuse readers or listeners should you casually drop it into conversation or insert it into your writing; noon is conventionally expressed as 12 p.m. or 12:00 p.m. and midnight as 12 a.m. or 12:00 a.m.

Content for this question contributed by Greg Anderson, resident of Pendleton, Umatilla County, Oregon, USA