If you know how to tell time, you probably know how to 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?
According to Dictionary.com, “a.m.” stands for the Latin phrase ante merīdiem, which translates to “before midday.” “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 a correct way to express “noon.”
As are many modern-day practices with Latin roots, the idea of splitting the day into two 12-hour halves 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, HuffPost reports.
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 it is OK to use “am/pm” or “AM/PM” elsewhere as long as you keep it consistent, Dictionary.com recommends.
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. And, 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.”