Ante Meridiem and Post Meridiem

AM PM Full Form in English, letter by letter

AM is Ante Meridiem (before midday) and PM is Post Meridiem (after midday), the two halves of the 12-hour clock.

AM PM Full Form in Indian Languages

Everyday speech in India just says AM and PM, or uses words like morning and evening. The formal terms below are what textbooks and forms use for Ante Meridiem / Post Meridiem.

Two Latin phrases hiding in your phone's clock

AM is ante meridiem, 'before midday'. PM is post meridiem, 'after midday'. The 'meridiem' part is the moment the sun crosses its highest point, so the whole system is built around noon. Everything from midnight up to that point is AM, and everything from noon round to midnight is PM.

Each half-day has twelve hours, numbered 12, 1, 2 and so on up to 11. That 12 at the start is doing the job of zero. This is why a clock goes from 11:59 AM straight to 12:00 PM, and why it feels odd the first time you think about it.

The 12 o'clock problem

This is where most arguments start. Noon is exactly midday, so it is neither before it nor after it, and strictly speaking neither AM nor PM fits. The usual convention is that 12 AM means midnight and 12 PM means noon, but plenty of people read it the other way, and the reverse has been used in the past.

Because a mix-up can mean a missed exam or train, several style guides tell writers to avoid both labels at that hour and write 'noon' and 'midnight' instead. Some timetables go further and print a midnight departure as 11:59 PM or 12:01 AM to dodge the confusion. If you are writing a form, a deadline or a ticket, 'noon' and 'midnight' are the safe words.

Why a day got split in two halves of twelve

The idea is very old. Egyptian sundials and water clocks, some dated to around 1500 BC, divided the period of daylight and the period of night into 12 parts each. The Romans did the same. Because the length of daylight changes with the seasons, those hours were not equal in length, and a summer day hour was longer than a winter one.

The first mechanical clocks of the 14th century often showed all 24 hours on one dial. In Northern Europe, dials with two sets of 12 numbers can still be seen at old cathedrals such as Wells and Exeter. During the 15th and 16th centuries the 12-hour dial became the standard for public use, and the 24-hour dial was kept for specialised jobs such as astronomical clocks. Most analogue watches today still use a 12-hour dial, where the hour hand goes round twice a day.

Who uses it, and who does not

The 12-hour clock is the everyday system in the United Kingdom, the United States, Canada, Australia, New Zealand, India, Pakistan and Bangladesh, among others. Even there, science, medicine, the military and transport often switch to the 24-hour clock, which is why a hospital chart may say 18:45 instead of 6:45 PM.

In India you meet both systems in the same day. An IRCTC train ticket and the railway timetable use the 24-hour clock, so a 22:10 departure is 10:10 PM. Exam admit cards, interview calls and shop timings usually use AM and PM. Most of the world outside English-speaking countries uses the 24-hour clock for writing, and the labels AM and PM are seldom used there. On a phone or computer, you can switch between the two in the date and time settings.

Quick conversions and writing it correctly

To go from 12-hour to 24-hour time, leave AM hours as they are, except that 12:xx AM becomes 00:xx. For PM hours from 1:00 to 11:59, add 12. So 7:15 AM is 07:15, 12:40 PM is 12:40, 3:30 PM is 15:30, and 11:45 PM is 23:45. Going the other way, subtract 12 from anything above 12:59 and mark it PM.

On style, there is no single rule. AM and PM may appear as 'am', 'AM', 'a.m.' or small capitals, depending on the style guide, and some ask for a space between the number and the label. What every guide agrees on is not to write AM or PM without a time in front of it, and to pick one format and stick to it across a document. In speech, people often say 'quarter past five' or 'half past five' rather than 5:15 or 5:30, and use phrases like 'in the morning' and 'at night' instead of the labels.

Everyday traps with AM and PM

Alarms are the commonest mistake. Setting an alarm for 6:00 when the phone is on PM wakes you up in the evening, so check the small AM or PM mark next to the time. The same trap appears when you book a flight or an online class across time zones, where a 'midnight' deadline might fall on one date for you and a different date for the organiser.

For deadlines, say the date as well. 'Submit by 12 AM on Friday' can be read as the start of Friday or the end of it. 'Submit by 11:59 PM on Friday' leaves no doubt, and a form that says 'noon' is clearer than one that says '12 PM'. If you are writing a notice for parents or students, the extra half line is worth it.

AM is ante meridiem, before midday, and PM is post meridiem, after midday. Midnight to noon is AM, noon to midnight is PM, and each half has twelve hours. The only real trap is the hour that starts each half, so write 'noon' or 'midnight' whenever someone could miss something because of a misreading.

AM PM Full Form: Common Questions