What Does AM and PM Stand For?

What does AM and PM stand for in Latin?

Ante means before. Post means after, as in "postscript" or "postwar." Meridiem comes from the Latin word for midday. English kept it in "meridian," the line the sun crosses at its highest point around noon.

What does AM stand for in time, and what about PM?

On a 12-hour clock, the numbers 1 through 12 repeat twice a day. The letters tell you which round you are in.

• a.m. runs from midnight through 11:59 in the morning. A 6 a.m. alarm is early morning.
• p.m. runs from noon through 11:59 at night. A 6 p.m. meeting is early evening.

The 12 o'clock problem

Noon is exactly midday, so strictly speaking it is neither before nor after it. In everyday American use, though, the custom is settled:

• 12 p.m. = noon, lunchtime.
• 12 a.m. = midnight, the start of a new day.

A simple way to remember it: one minute after noon is 12:01 p.m., and the afternoon keeps the p.m. label. One minute after midnight is 12:01 a.m., the first minute of the morning.

Because people still mix these up, many schedules, tickets and contracts avoid the problem by writing "12 noon" and "12 midnight," or just "noon" and "midnight." For a deadline, also name the date: "midnight at the end of Friday" is clearer than "12 a.m. Friday."

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

What does a.m. and p.m. stand for once you add the periods? The same Latin phrases. Only the look changes from one style to another.

• Lowercase with periods is the common style in US books and newspapers: 9:30 a.m.
• Capitals without periods, AM and PM, are standard on digital clocks, phones and many signs.
• Leave a gap after the digits: 4 p.m., not 4p.m.
• Skip doubles. "8 a.m. in the morning" says the same thing twice. Write "8 a.m." or "8 in the morning."

If a sentence ends with p.m., the period does double duty: "The store closes at 9 p.m." Only one dot is needed.

AM and PM vs the 24-hour clock

Many countries, hospitals and the military skip the letters and count the hours from 0 to 23. That style is often called military time in the United States.

To convert a p.m. time, add 12 to the hour, except for noon. Time zones add their own letters, such as GMT or ET after the time.

Other AM and PM meanings

Not every AM or PM is about the clock.

• AM radio: amplitude modulation, the older broadcast band.
• PM in politics: prime minister, the head of government in many countries.
• PM in texts: private message, as in "PM me the address."
• PM at work: project manager or product manager.

The setting usually makes it clear. You will find more short forms like these on the famous acronyms page, and other Latin leftovers such as P.S. and i.e. have their own pages.