What Do “A.M.” and “P.M.” Stand For?

Many of the English words and abbreviations we use every day have origins in Latin. Some of those common abbreviations are a bit perplexing. The tiny lb, short for pound, comes from the Latin Libra pondo. And No. for number comes from numero, which explains that random-seeming o at the end. So what about a.m. vs. p.m.?

These two everyday abbreviations come from Latin as well. Keep reading, and we’ll break it down.

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What are a.m. and p.m.?

In the United States and several other English-speaking countries, these abbreviations are essential for properly setting alarms and scheduling appointments. And yet many people still don’t know what a.m. and p.m. stand for. 

If you’re one of them, wonder no more. 

The meaning of a.m.

The abbreviation a.m. stands for ante meridiem, a Latin phrase that means “before noon” or “before midday.” That’s why, in the 12-hour system, all times from midnight until noon get this designation. 

The meaning of p.m.

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The abbreviation p.m. stands for post meridiem, meaning “after noon” or “after midday.” This applies to all times from 12 p.m. to 11:59 p.m. While the origin of the terms can be traced back to the days of sundials, English-speaking countries have been using a.m. and p.m. since the 17th century. The Latin word post has also made its way into our language on its own, synonymous with after.

How do you punctuate a.m. and p.m.?

Since they’re abbreviations, do you need to capitalize a.m. and p.m.? When it comes to formatting, the grammar “rules” aren’t as strict as you might think. While popular stylebooks recommend using lowercase with periods (a.m. and p.m.), other formats are acceptable in informal writing, as long as you’re consistent, so feel free to go with uppercase, lowercase or even small caps if you have that formatting option. (Oh, and when you’re using periods and the abbreviation ends a sentence, you don’t need an extra period—one will do.)

How do a.m. and p.m. work in military time?

If you’re using military time, or the 24-hour system, you don’t need a.m. and p.m. at all, because there are no duplicate numbers. After 12:59 p.m., instead of starting over at 1:00,  the next hour becomes 13:00. The hour just after midnight is 00:00, eliminating the need for a second 12:00.

If you’re not familiar with military time, here are some tips:

• From midnight (12 a.m.) until 12:59 a.m., swap the 12 for a zero.
• Any hour that begins with a zero (for example, 01:00) corresponds to a.m. hours in the 12-hour system.
• To identify how a post-12:59 p.m. time translates into military time, add 12 to the hour number.
• To convert military time back to a 12-hour format, subtract 12 from any hour after 12:00. For example, 15:45 would become 3:45 p.m. Scratching your head at what time 21:39 is? Subtract 12 from the hour to get 9:39 p.m.

What about noon and midnight?

One of the most confusing parts of the a.m. vs. p.m. system is midnight: Is it 0:00 or 24:00? Well, this can be a good way to differentiate between the preceding midnight and the upcoming midnight, or the beginning of a certain day and the end of that day.

The confusion that can arise from having two of every “o’clock” in a day is definitely a solid argument in favor of using military time. So, in the 12-hour system, which 12 o’clock is which?

Technically, 12 o’clock at night—12 a.m.—is 12 hours after the previous noon and 12 hours before the next. But is it before or after noon? And of course, 12 p.m. is noon, so it may seem silly to designate it “before” or “after.” To avoid confusion, most English-speaking countries use 12 a.m. for midnight, since it marks the start of a new day and comes before noon. Switching directly from 12 p.m. to 12:01 a.m. would only add to the confusion. So noon becomes 12 p.m. by default.

Though if you’re linguistically opposed to calling noon “after noon,” there’s actually another Latin abbreviation you can use. The a.m./p.m. system does have a specific abbreviation for noon—just the letter M, short for meridiem, which would come after “12” and only refer to noon. That said, the “12 M” format is virtually obsolete. 

How can you teach your child the a.m. vs. p.m. system?

Kids often learn more easily when a concept is introduced through a fun song or rhyme. Here’s one to help them understand a.m. vs. p.m.: 

a.m. is morning, the sun starts to rise,
Time to wake up and open your eyes.
At noon, p.m. begins its stay,
Lunch, then dinner, then sleep ends the day!
At midnight, a.m. comes back to play!

In today’s digital world, when our devices constantly display the time, knowing the difference between a.m. and p.m. might seem less than essential. But learning about the origins of words is always worthwhile, and it makes for great trivia in conversations. 

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Sources:

• Dictionary.com: “What Do ‘A.M.’ and ‘P.M.’ Stand For?”
• Military Time: “What Do AM and PM Stand For?”
• Time and Date: “What Do AM and PM Stand For?”
• Encyclopedia Britannica: “12-hour clock: time convention”
• Montana State University: “Standard Meeting Patterns and Military Time”

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