Most of us will have been made aware from a young age that “am” and “pm” refer to the first half and the second half of the day, or morning and afternoon/evening – but what do those letters actually stand for?

Take a look at Latin

Some people might think Latin is a dead language, but take a quick look around and its influence is everywhere, including in how we describe time. “am” stands for the Latin phrase ante meridiem, which translated to English means “before midday”. On the other hand (well, same hands if you’re looking at a 12-hour clock), “pm” stands for post meridiem, meaning “after midday”.

If you fancy whipping out a bit of pedantry the next time you hear someone say 12 am or 12 pm, this means that you can tell them that doing so isn’t technically correct – 12 pm can’t be after midday because it’s, well, midday. Royal Museums Greenwich recommends sticking to 12 noon or 12 midnight.

All that being said, people might find you a bit annoying if you start pointing this out at every opportunity.

To punctuate or not to punctuate

According to the Chicago Manual of Style, there are a total of six ways to write abbreviate ante meridiem and post meridiem: all caps with periods, all caps without period, small caps with periods, small caps without periods, lowercase with periods, and lowercase without periods.

For a long time, the use of capital letters with the inclusion of periods was the preferred option for many formal style guides, including Chicago. Though that might remain in some academic settings or publishing houses, for example, any which way you want to write it is generally considered acceptable nowadays – just be consistent with what you use.

The less confusing way

If there are concerns about accidentally turning up at the airport at 4 pm when your flight was actually at 4 am (a time worth missing a flight for, to be fair), then you might just want to switch to using the 24-hour system.

Though your pals might rinse you for pretending to be in the military, it can be a much less confusing approach. For example, instead of midnight being 12 am, it simply becomes 00:00, which makes a lot of sense considering it’s the start of the day. 

Where the territory becomes slightly more unfamiliar is after 1pm, which instead becomes 13:00, and so on until midnight, which actually still has its problem – should it be 00:00 or 24:00? We’ll leave that one up to you to debate.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.