"Dave Cantor" <Dave@Cantor.mv.com> writes:
Using 12:00 M for noon is what I learned in junior high school, back in the 1950's. We were also taught that midnight was properly called 12:00 p.m. (it is the last minute of the ending day, and is therefore 12 hours after noon, which is what 'p.m.' literally means).
Thanks for your recollections. Older editions of the Chicago Manual of Style (CMS) indeed recommended that '12 m.' means noon and '12 p.m.' means midnight. However, this recommendation was removed sometime during the 14th edition of the CMS. Here's my source: Over the years, we issued about ten printings of the fourteenth edition. Each time we reprinted, we corrected errors that had been pointed out since the last printing. In section 8.48 we wanted to eliminate two examples that seemed more confusing than helpful (and 12:00 P.M. is in fact noon). For the fifteenth edition, we have made several clarifications, including the recommendation that numerals not be used for midnight and noon except in the twenty-four-hour system. Note also that the fifteenth recommends writing "a.m." and "p.m.," though the more traditional small capitals are still accepted (but now without periods: AM and PM). <http://www.press.uchicago.edu/Misc/Chicago/cmosfaq/cmosfaq.CMS.html>. So, I don't know the origin of the tradition you were taught in junior high school, but the CMS is now saying that it was an error! I doubt whether that tradition came from the Romans themselves, since the original Latin tradition was that 3 a.m. was three hours before noon (which is what "a.m." literally means), so the Roman 3 a.m. was our 09:00. Hence, in the original Latin tradition, I guess '12 a.m.' and '12 p.m' would both have meant midnight. Wikipedia says that one should use '12 m.n.' instead, where the 'm.n.' means 'media nox', but I'm dubious of this as well. I suspect that the actual Romans almost invariably wrote either 'meridies' for noon, or 'media nox' for midnight, and didn't bother with the 'xii'. For a (modern) example of this, the motto of the RAF No. 409 Nighthawk Squadron during World War II was "Media nox meridies noster" ("Midnight is our noon") <http://www.rcaf.com/squadrons/400series/409squadron.php>. The RAF didn't bother with 'xii', and I'll bet the Romans didn't either, and that "12 m." and "12 p.m." are neologisms, and confusing ones as well.