What Do AM and PM Stand For?

We use these abbreviations all the time, but do we actually know what they mean?

Many of the English words and expressions we use daily can trace their origins back to Latin, and this is true for some of the English language’s more perplexing abbreviations. “Lb,” the seemingly out-of-the-blue abbreviation for “pounds,” comes from the Latin phrase “Libra pondo.” “No.” as an abbreviation for the decidedly O-less word “number” comes from the Latin “numero.” And, if you’ve ever wondered, “What do AM and PM stand for?” look no further: These common abbreviations come from Latin too. Find out where other common abbreviations you use all the time come from as well.

What do AM and PM stand for?

In the United States and several other English-speaking countries, we use these abbreviations all the time. The correct use of them makes or breaks our alarm-clock settings. And yet many people can’t correctly answer the question “What do AM and PM stand for?”

Well, if you’re one of those people, wonder no more. “AM” stands for “ante meridiem.” The Latin phrase “ante meridiem” means “before noon” or “before midday.” That’s why, in the 12-hour system, all times from midnight onward use this designation. PM stands for “post meridiem,” meaning “after noon” or “after midday,” and as such applies to the times from noon onward. The Latin word “post” has also made its way into our language on its own, synonymous with “after.” To unlock some more time mysteries, find out why we have daylight saving time.

What about noon and midnight?

One of the most confusing aspects of the AM-PM system, once you’ve answered the question “What do AM and PM mean,” is how it accounts for midnight and midday. The confusion that can arise from having two of every “o’clock” a day is definitely a solid argument in favor of using military time. So, in the 12-hour system, which 12:00 is which?

Technically, 12:00 at night, which we know as “12 AM,” is exactly 12 hours after the previous noon and before the coming noon, so does it count as “before” noon or “after” it? And, of course, 12:00 PM is noon, so it may seem silly to designate it “before” or “after.” English-speaking countries parse it out by using “12:00 AM” to refer to midnight, since midnight starts the new day, so it can be considered “before noon” of the same day. Not to mention it would probably be even more confusing if time went from 12:00 PM to 12:01 AM. So noon then becomes 12 PM by default.

Though, if you’re deeply linguistically opposed to calling “noon” “after noon,” there’s actually another Latin abbreviation you can use. The AM/PM system actually does have a specific abbreviation for noon—just the letter “M,” short for “meridiem,” which would come after “12” and only refer to noon. Haven’t heard of it? Well, for better or worse, the “12 M” designation for “noon” is quite rare and has been pretty much lost to antiquity. If the answer to the question “What do AM and PM mean” surprised you, find out the true meanings of 31 common acronyms you never knew.

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Meghan Jones
Meghan Jones is a Staff Writer for RD.com who has been writing since before she could write. She graduated from Marist College with a Bachelor of Arts in English and has been writing for Reader's Digest since 2017. In spring 2017, her creative nonfiction piece "Anticipation" was published in Angles literary magazine. She is a proud Hufflepuff and member of Team Cap.