You may think this simple question has a simple answer (either 12 a.m. or 12 p.m.), but it’s not quite so.
The abbreviation a.m. stands for the Latin phrase ante meridiem, and p.m. stands for post meridiem. In Latin, ante means before; post means after; and meridies means noon, or midday.
Midday is neither before noon (a.m.) nor after noon (p.m.). It’s m. (abbreviation for meridies).
So, it would be illogical to write 12 a.m. or 12 p.m. to mean noon.
What experts say about 12 a.m. vs. 12 p.m.
According to the New Oxford Style Manual, noon is 12 p.m.: “Correctly, 12 a.m. is midnight and 12 p.m. is noon”. The same source says, “since this is not always understood, it may be necessary to use the explicit 12 midnight and 12 noon.”
In contrast, the Chicago Manual of Style says noon can be referred to as “12:00 m. (m. = meridies)”, though very few people use this form. According to the same style guide, “12 p.m. is ambiguous, if not illogical”.
The author of Garner’s Modern English Usage writes, “To refer to noon as either 12 AM or 12 PM is not just logically and astronomically wrong, but ambiguous as well.”
This freelance editor’s advice
- Avoid using 12 a.m. or 12 p.m. Say noon when you mean middle of the day.
- Use the 24-hour system of expressing time. In this system, common in scientific contexts, 12:00 means noon (and 24:00 means midnight).
- In informal contexts, you can use 12 noon if you want, but I would avoid it in formal writing.
In summary, noon is meridies in Latin, but 12 m. to mean noon is rarely used. If you are writing a scientific paper, use the 24-hour clock. In most other contexts, refer to noon as noon to prevent confusion. Finally, if your publisher has an editorial style guide, follow it—or ask your editor for advice.
Related posts:
- Do you hyphenate Latin phrases?
- Is “et al.” singular or plural?
- How to choose the indefinite article (a or an) before acronyms
Last revised on 16 July 2022
