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From several sources, including english.stackexchange.com, one should write 3 p.m. instead of 3 PM.

How did the all-capitals variant appear, and especially why? Is it because with typewriters and in terminals, PM uses half as much space than a.m.?

Community
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Arseni Mourzenko
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  • I think you have found already found your answer: 'Because the initials in question are of Latin origin ("post meridian"/"ante meridian")' – Dog Lover Jun 10 '15 at 12:31
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    You should probably change your question to why "AM"/"PM" came into play (as opposed to "a.m."/"p.m.") to avoid running the risk of having your question closed as a duplicate. – Dog Lover Jun 10 '15 at 12:32
  • Why? It's two characters less to type. People write w8, 4 and r to avoid typing too many characters, so why bother with the extra dots? In many situations, people want their message to be conveyed, and they don't really care if someone wants to judge their message as if it were a literary work. Why the capitals? I guess to indicate, in absence of the dots, that it is an abbreviation (you could confuse am with the verb). Why do these things appear? Because people find them easy, it turns out they are perfectly understandable, so there is hardly any reason to use the less-easy way. – oerkelens Jun 10 '15 at 12:55
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    "The terms "a.m." and "p.m." are abbreviations of the Latin ante meridiem (before midday) and post meridiem (after midday). Depending on the style guide referenced, the abbreviations "AM" and "PM" are variously written in small capitals ("ᴀᴍ" and "ᴘᴍ"), uppercase letters ("AM" and "PM"), or lowercase letters ("am" and "pm")." Wikipedia http://en.wikipedia.org/wiki/12-hour_clock#Typography – Kris Jun 10 '15 at 13:23
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    "one should write 3 p.m. instead of 3 PM" -- not necessarily. – Kris Jun 10 '15 at 13:24
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    "why" questions have almost no meaning. – Fattie Jun 10 '15 at 13:27
  • No, one should write 15:00 instead of 3 PM. – Hot Licks Jun 10 '15 at 16:46

2 Answers2

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I believe the use of AM and PM increased during the early days of computer systems. Back in character-mode days, one had to be especially parsimonious with screen and page "real-estate". Uppercase AM and PM without punctuation used only two characters, and there was also no space between the time value and the AM or PM value. It was often common to see all uppercase.

But 6pm seems to be on the ascendant.

TimR
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The all-capitals variant is probably borrowed from AM/FM radio. (These are abbreviations for amplitude modulation and frequency modulation}

Oldbag
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