Q: The parking signs in my town refer to noon as 12 p.m. Since “p.m.” stands for “post meridiem” (“after noon” in Latin), can 12 p.m. be used for noon itself?
A: The simple answer is yes, but we’d advise against it. By convention, the term “12 p.m.” is used for noon in countries like the US with a 12-hour clock.
For those who argue that noon and midnight are neither a.m. nor p.m., we can only cite the American Heritage Dictionary of the English Language (online 5th ed.), which has the following usage note with its entry for “PM”:
“By convention, 12 AM denotes midnight and 12 PM denotes noon. Because of the potential for confusion, it is advisable to use 12 noon and 12 midnight where clarity is required.”
(The print version of the dictionary’s 5th edition, which isn’t as up-to-date as the online version, has “by definition” instead of “by convention.”)
We agree that “12 a.m.” and “12 p.m.” are confusing and should be avoided, but one could also argue that “12 noon” and “12 midnight” are redundant. Why not simply say “noon” and “midnight”?
You may be interested in knowing that “meridiem” actually means midday in Latin, and that the terms “noon” and “midday” have not always been synonymous in English.
As the Oxford English Dictionary points out, the word “noon,” dating back to the year 900, originally meant “The ninth hour of the day, reckoned from sunrise according to the Roman method, or about three o’clock in the afternoon.” By the 14th century, according to the OED, the word “noon” had come to mean 12 o’clock.
Although dictionaries usually define “midday” as the middle of the day or noon, it’s often used more loosely than the word “noon.”
The entry for “noon” on Wikipedia, for example, describes midday as “the period of early afternoon, beginning at noon and lasting until mid-afternoon.”
Finally, in case you’re wondering, we prefer to lowercase and punctuate the terms “a.m.” and “p.m.,” as recommended in The Chicago Manual of Style (16th ed.).
Well, that’s enough noon-sense for now.
[Note: This post was updated on Nov. 16, 2015.]