What AM and PM Stand For and Where They Come From

AM stands for ante meridiem, Latin for “before midday,” and PM stands for post meridiem, Latin for “after midday.” The A in AM is ante, which means “before,” so AM runs from midnight to noon and PM runs from noon to midnight.

Last updated: October 29, 2026

AM vs. PM at a glance

The 12-hour clock splits each day into two halves. Noon is the dividing line. The table below shows what each abbreviation stands for and which hours it covers.

Abbreviation	Full Latin	Literal meaning	Hours covered
a.m.	ante meridiem	before midday	12:00 midnight to 11:59 in the morning
p.m.	post meridiem	after midday	12:00 noon to 11:59 at night

You can also hear the two phrases this way:

• ante meridiem: AN-tee muh-RID-ee-um (/ˌænti məˈrɪdiəm/)
• post meridiem: POHST muh-RID-ee-um (/poʊst məˈrɪdiəm/)

In everyday speech, nobody says the Latin. You just say the letters: “Meet me at nine a.m.”

What does the A in AM stand for?

The A stands for ante, the Latin word for “before.” That’s it. The M stands for meridiem, “midday.”

Many people guess that AM means “after midnight.” That’s a backronym, a phrase invented to fit letters that already existed. It happens to work, because the AM hours do come after midnight. But it isn’t where the abbreviation comes from.

The same goes for PM as “past midday” or “past midnight.” The P really stands for post, Latin for “after.”

The Latin, piece by piece

Each part of the phrase has relatives you already use in English.

Ante: before

Ante shows up in plenty of English words:

• antebellum: before the war, often used for the pre-Civil War American South
• anteroom: a small room before a larger one
• antecedent: something that comes before

Post: after

Post is even more familiar. Think of postwar, postpone and postgraduate. The P.S. at the end of a letter is short for post scriptum, “written after.” That’s the same post you find in p.m.

Meridiem: midday

Meridiem is a grammatical form of the Latin noun meridies, meaning “midday” or “noon.” Latin speakers also used meridies to mean “south.” That makes sense once you picture it. In Rome, the midday sun sits in the southern sky.

The word is usually explained as coming from medius (middle) and dies (day). So the “middle of the day” became one word.

English borrowed the same root as meridian. A meridian is an imaginary line running north to south across the globe. When the sun crosses your local meridian, it reaches its highest point. That moment is solar noon. So “before midday” and “after midday” originally meant before and after the sun crossed that line.

If you enjoy unpacking roots like this, the etymology of the word is often hiding in plain sight inside longer words. Our piece on the prefix tachy- takes the same approach with a Greek root.

Why noon is the dividing line

Splitting the day at noon is ancient. It comes from watching the sun.

The Egyptians divided daylight into 12 parts and the night into 12 more. They tracked daytime hours with sundials and night hours with water clocks and the stars. The Romans also counted 12 hours of daylight. Their hours stretched in summer and shrank in winter, because the daylight itself changed length.

Midday was the easiest moment to spot. The sun is at its highest. Shadows are at their shortest. So it became the natural hinge of the day, with one set of hours before it and one after.

When mechanical clocks spread across medieval Europe, many used 12-hour dials. That kept the before-noon and after-noon split alive. Latin was the language of scholarship at the time, so the Latin labels stuck.

A quick detour: where “noon” comes from

The word noon has a twist in its history. It comes from Latin nona hora, “the ninth hour.” Counting from sunrise, the ninth hour fell around 3 in the afternoon.

In the medieval Church, a prayer service called None was held at that ninth hour. Over time, the service and the meal tied to it drifted earlier in the day. Eventually “noon” settled on midday. So noon once meant mid-afternoon.

The 12 a.m. and 12 p.m. problem

Here’s where the system gets awkward. Noon is neither before nor after midday. It is midday. Midnight sits on the line between two days.

By common convention:

• 12 p.m. means noon
• 12 a.m. means midnight

Plenty of people still mix these up. A ticket that says “12 a.m. on Friday” can confuse someone who isn’t sure whether that means the start or the end of Friday.

The U.S. National Institute of Standards and Technology (NIST) treats 12 a.m. and 12 p.m. as ambiguous. It recommends writing 12 noon and 12 midnight instead.

How to write a.m. and p.m. correctly

There’s no single rule. There are a few common styles, and the main thing is to stay consistent within one piece of writing.

Style	Example	Where you see it
Lowercase with periods	9 a.m.	AP style, Chicago style, most US publishing
Uppercase, no periods	9 AM	Digital clocks, apps, signs
Small capitals	9 AM	Some books and formal typesetting
Lowercase, no periods	9am	Common in British writing and casual text

A few habits make your writing cleaner:

• Put a space before the abbreviation in formal writing: 7 p.m., not 7p.m.
• Don’t double up. “10 a.m. in the morning” says “morning” twice. Choose one.
• Drop the :00 if you like. “3 p.m.” and “3:00 p.m.” both work. Just keep the same choice throughout.
• Don’t mix in o’clock. Write “5 o’clock” or “5 p.m.,” not “5 o’clock p.m.”
• Use numbers, not words, with a.m. and p.m.: “6 p.m.,” not “six p.m.”

AM, PM and the 24-hour clock

The 24-hour clock does away with AM and PM entirely. It counts the hours from 00 to 23. Most of the world uses it in writing, and so do armed forces, hospitals, airlines and train timetables. The reason is simple. There’s no chance of confusing 7 in the morning with 7 at night.

In the United States, people often call it military time. There, 1500 is spoken as “fifteen hundred hours.”

Converting from AM/PM to 24-hour time

Use these three rules:

1. For a.m. times from 1:00 to 11:59, keep the number as it is. 9:30 a.m. becomes 09:30.
2. For p.m. times from 1:00 to 11:59, add 12 to the hour. 4:15 p.m. becomes 16:15.
3. For the 12 o’clock hours, treat them as exceptions. 12:00 noon is 12:00. 12:00 midnight is 00:00.

To go back the other way, subtract 12 from any hour above 12 and add “p.m.”

12-hour clock	24-hour clock
12:00 a.m. (midnight)	00:00
1:00 a.m.	01:00
6:45 a.m.	06:45
11:00 a.m.	11:00
12:00 p.m. (noon)	12:00
1:00 p.m.	13:00
5:30 p.m.	17:30
9:00 p.m.	21:00
11:59 p.m.	23:59

Practice questions

Try these before checking the answers.

1. What is 8:20 p.m. in 24-hour time?
2. What is 14:05 on the 12-hour clock?
3. Your train leaves at 00:40. Is that a.m. or p.m.?
4. A shop opens at 0930 hours. What time is that?
5. What does the a in a.m. stand for?

Answers:

1. 20:20 (add 12 to 8)
2. 2:05 p.m. (subtract 12 from 14)
3. a.m. It’s 12:40 a.m., forty minutes after midnight.
4. 9:30 a.m.
5. Ante, Latin for “before,” as in before midday.

How other languages say before and after noon

English borrowed Latin. Other languages built their own versions, and some line up neatly with the Latin idea.

English	Language	Translation	Note
a.m.	Spanish	a. m. / de la mañana	Spanish also writes “a. m.” from the same Latin
p.m.	Spanish	p. m. / de la tarde, de la noche	“Tarde” is afternoon, “noche” is night
in the morning	French	du matin	Written times usually use the 24-hour clock
in the evening	French	du soir	“Après-midi” covers the afternoon
a.m.	German	vormittags	Literally “before midday”
p.m.	German	nachmittags	Literally “after midday”
a.m.	Japanese	午前 (gozen)	Literally “before noon”
p.m.	Japanese	午後 (gogo)	Literally “after noon”

The Japanese pair is a lovely parallel. The character 午 refers to the hour of the Horse in the traditional Chinese zodiac clock, the two-hour block around midday. So gozen and gogo mean “before the horse hour” and “after the horse hour.” That’s the same logic as the Latin, built on a different system.

Other things AM and PM stand for

The same letters carry other meanings depending on context:

• AM radio: amplitude modulation, a way of carrying a signal on radio waves
• A.M.: Master of Arts, from Latin artium magister, used by some universities
• PM: prime minister
• PM: post-mortem, an examination after death
• PM: private message, especially in online forums and chat apps

That last one belongs with other chat shorthand. Like S/O and W/E, “PM me” is something you’ll see in texts and on social media, and it has nothing to do with the time of day.

FAQ

What does AM and PM stand for?

AM stands for ante meridiem, Latin for “before midday.” PM stands for post meridiem, Latin for “after midday.”

What does the A stand for in AM?

The A stands for ante, the Latin word for “before.” It doesn’t stand for “after,” even though “after midnight” is a popular guess.

Is 12 a.m. midnight or noon?

By convention, 12 a.m. is midnight and 12 p.m. is noon. Because so many people mix them up, it’s clearer to write “noon” or “midnight.”

Should I write AM, am or a.m.?

Any of them is acceptable if you stay consistent. US style guides such as AP and Chicago prefer lowercase with periods: a.m. and p.m. Apps and digital clocks usually show AM and PM.

Why do we use Latin for time?

Latin was the shared language of science, law and the Church in Europe for centuries. Clockmakers and scholars used Latin terms, and the abbreviations survived long after people stopped speaking the language.

Is “ante meridian” correct?

The standard Latin form is meridiem. “Meridian” is a related English word for a north-south line on a map. You will see “ante meridian” in print, but ante meridiem is the form dictionaries give.

About this article

RP

Written by Rosalind Petrie

Rosalind Petrie writes Star Dictionary's guides to word meanings, origins, idioms and usage, from everyday definitions and pronunciation to commonly confused words and grammar questions. She checks each entry against established dictionaries, historical citation records and usage guides, and she cares about giving readers clear examples they can actually use in a sentence.

• Fact-checked by Maureen Talbot before publication.
• Published October 29, 2026