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Sir,

a.m. or A.M. which one is correct?

In my book , The full form of a.m. is written as antemeridian.

But when I consulted a dictionary,I found it as " ante meridiem".

antemeridian or ante meridian or ante meridiem or antemeridiem

which one is correct?

Thanks.
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As far as the abbreviation is concerned, there seems to be no consensus, so [url="http://www.onelook.com/?w=a.m.&ls=a "]TAKE YOUR PICK[/url]. I myself usually use 'a.m.' and 'p.m.', but evidently all permutations-- upper or lower case, with or without the punctuation-- are acceptable in some quarters.

It is Latin for 'before the zenith', and as such should be written 'ante meridiem'-- the use of the English 'meridian' mixes the two languages in an ugly fashion.
In addition,

ante meridian or antemeridian, no matter what the written forms look like, is treated as one word linguistically: ante- is a prefix and meridiem is the noun it modifies.

Orthographically, the pair ante + meridian is viewed as two separate words, which is the reason periods are used (a.m.), but linguistically, the pair is viewed as a single unit. The prefix ante- cannot stand on its own as a word. It needs to attach itself to a base, or root to realize its meaning. Its dual status is the reason that it's traditionally written as an abbreviation but with more than one period: a.m. Arguably, it looks like an acronym (A.M./AM and P.M./PM), but there are reason why it's not. Allow me to explain.

To reduce the length of a single word, use an abbreviation. Abbreviations are formed by taking the first few letters, and adding a period at the end; e.g., Company => Co. The prefix ante- is part of a word.

To reduce the length of two or more words, use an acronym. Acronyms are formed by taking the first letter of each word, and separating them with a period (the periods are optional); e.g., U.S.A ~ USA. Moreover, acronyms represent names, proper nouns. The prefix ante- is not a word, and it's not a proper noun.

Speakers often use A.M./AM and P.M./PM, but note, if you use those forms, you're indirectly saying this: the prefix "ante-" is a proper noun. Now, if you believe that, then by all means use A.M.

Please note,
Here's a trick that might help you to differentiate an abbreviation from an acronym: an abbreviation has only one period, whereas an acronym has more than one period or no periods at all. There are exceptions, but only one that I know of: the abbreviations a.m. and p.m.
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Your explanation is excellent for English, Casi, but this is a Latin phrase we are looking at: ante ('before') plus meridiem ('midday')-- it's two words, a Latin prepositional phrase (as is 'post meridiem'). The abbreviation, 'a.m.' is a simple abbreviation of two Latin words, like 'e.g.' or 'i.e.'-- are those acronyms, or what do we call them, beyond 'abbreviations'?

Now if it comes all the way into English (as it seems to be threatening to do), and turns into 'ante-meridian', the prefix will no longer become separable, I would think.
Thanks, MM, but don't the words "dual status" (See my post) take that into account? That is, I agree with you 100%.

The problem I see, as do you, is with the abbreviation having more than one periods. (The added exceptions, e.g. and i.e., are wonderful, by the way. Got any more?)
Well, I thought of 'etc' , 'et al', and 'viz', but immediately abandoned them, as undermining my argument...
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n.b.

q.e., q.v.

(q.e.d.)

MrP
A.M. and P.M. are abbreviations for ante meridian and post meridian. They refer to morning (A.M.) and afternoon, evening and night (P.M.). They are commonly written as am or pm. They can be capitalized, lower case, with or without punctuation. The most common in American usage are am and pm, as in 3:00am or 12:30pm. A.M is the hours from 12:00 midnight to 11:59am. P.M. is the hours from 12:00 noon to 11:59pm.
AnonymousA.M. and P.M. are abbreviations for ante meridian and post meridian. They refer to morning (A.M.) and afternoon, evening and night (P.M.). They are commonly written as am or pm. They can be capitalized, lower case, with or without punctuation. The most common in American usage are am and pm, as in 3:00am or 12:30pm. A.M is the hours from 12:00 midnight to 11:59am. P.M. is the hours from 12:00 noon to 11:59pm.
Consider Mister M's explanation, Anon:


It is Latin for 'before the zenith', and as such should be written 'ante meridiem'-- the use of the English 'meridian' mixes the two languages in an ugly fashion.

MrP
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