What do a.m. and p.m. stand for?
Ever wondered about this?
speakers know that a.m. refers to the hours from midnight and noon, and p.m.
to the hours from noon and midnight. But what do these ubiquitous abbreviations
The term we
associate with the morning, a.m., is an abbreviation the Latin phrase ante
merīdiem meaning “before midday”; p.m. is an abbreviation of post
merīdiem, meaning—you guessed it—“after midday.”
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.
writing, it is best to lowercase both a.m. and p.m. and retain
the periods (as opposed to writing them AM/PM or am/pm);
however, should you choose to use a different format, the most important rule
is to be consistent throughout your piece.
Note that when a.m. or p.m.
end a sentence, as in The train is arriving at 10 p.m., it is not
necessary to place an additional period at the end. And, for our final writing
tip, since these abbreviations specify an hour’s relation to noon, it is not
necessary to use morning, evening, night, or o’clock
Clancy's comment: There ya go.