What Do A.M. and P.M. Stand For?

If you know how to tell time, you probably understand and use a.m. and p.m., and you might even know the terms come from Latin phrases. But do you know what exactly those phrases are, or what they mean in English?

What do a.m. and p.m. mean?

According to Dictionary.com, a.m. stands for the Latin phrase ante merīdiem, which translates to “before midday.” The term p.m., on the other hand, is an abbreviation of post merīdiem, or “after midday.” Have you ever noticed somebody write “12 m.” or “12:00 m.”? Though uncommon, it’s technically the correct way to express noon. (Noon, by the way, actually used to refer to 3 p.m.)

As with many modern-day practices with Latin roots, the idea of splitting the day into two 12-hour chunks is very, very old. So old, in fact, that we don’t know exactly how it became a worldwide habit; its history dates back to ancient Egypt and Mesopotamia at the very least.

How should you write a.m. and p.m.?

If you’re writing a.m. or p.m. in anything formal—an academic paper, published article, or fan letter to your favorite Mental Floss author, for example—you should stick to lowercase letters and include the periods, like we’ve done throughout this piece. But as Dictionary.com notes, it is OK to use “am/pm” or “AM/PM” elsewhere, as long as you keep it consistent.

From legal terminology to medical diagnoses to Harry Potter spells, you can find Latin in plain sight practically everywhere you look, not to mention the plethora of English words you may be speaking without even realizing we took them straight from the so-called dead language. If this article has inspired you to learn the Latin behind other common abbreviations, you can start here with the often-misunderstood difference between “i.e.” and “e.g.”

Are there other ways to say “a.m.” and “p.m.”?

If saying “a.m.” or “p.m.” is getting a little boring, there is slang from the 1910s that you can turn to: Early 20th-century telephone operators used pip emma to signal p.m., and ack emma to signal a.m.—so if someone said they planned to catch the train at seven o’clock pip emma, they meant 7 p.m. Eventually, both phrases (which, according to Green’s Dictionary of Slang, originated with the military) entered the public lexicon as alternatives to afternoon and morning.

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A version of this story ran in 2022; it has been updated for 2024.