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Which of the following are correct?

      right   wrong
ID    [x]     [ ]
i.d.  [x]     [ ]
I.D.  [ ]     [ ]
Id    [ ]     [ ]
I.d.  [ ]     [ ]
id    [ ]     [ ]

Marked the ones i found in dictionary.com

And what are the correct pluralized versions?

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3  
I don't have the time right now to write up a full answer, but ID, i.d., I.D., are all easily understood, the ones with different case aren't, and id could give someone pause given the similarity to the first word in id est, a Latin phrase somewhat commonly used in English. –  waiwai933 Apr 14 '11 at 19:55
    
@waiwai933: id est isn't used often at all outside its abbreviated form i.e., so I'd be more worried about confusion with the (also rather uncommon) word id. –  Jon Purdy Apr 15 '11 at 1:16
    
Suggestion: Update the check marks according to correct/chosen answer. –  M. Joanis Jan 8 '12 at 10:24
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5 Answers 5

up vote 4 down vote accepted

Both the NOAD and the OED report that ID is an abbreviation for identity, identification.

They weren't carrying any ID.
I lost my ID card.

The term id is used in psychoanalysis, and Id is a variant spelling of Eid.

In some contexts, id could be understood as ID, for example in the phrase the user id used when talking of a CMS.

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2  
ID would be least confusing. –  mgb Apr 14 '11 at 20:25
3  
Although I am still waiting for my official government ego card. –  mgb Apr 14 '11 at 23:47
    
@mgb: Somehow I feel that's outside government jurisdiction. –  Jon Purdy Apr 15 '11 at 1:19
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My gut says "ID", but luckily, so does empirical evidence: Google NGrams Viewer overwhelmingly employs "ID", and Canada's Termium terminological database accepts "ID" as correct (scroll down).

enter image description here

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None are intrinsically "incorrect" (whatever that really means), but the most common in everyday writing would be "ID" or "I.D.". Especially in the UK, it's really not common to put full stops in this kind of word/abbreviation in real-life usage-- I almost think this seems to be something of a primary-school-teacherism.

I can't recall seeing "i.d." or "I.d.".

The other variants ("Id" and "id") wouldn't usually be used in normal writing contexts, but are quite common in some computing contexts. For example, a common convention for naming functions/methods is to capitalise the first letter of "words" and treat abbreviations as normal "words". With that plus the convention of starting variable names with small letters, you would end up with things like:

getTokenId()

idOfUser

But obviously these are quite domain-specific uses.

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Yes - I.D. seems to imply that it's an acronym, i.e. two words, whereas Id. implies contraction. Because it's essentially a nickname for something pronounced the way it's spelt, I'd go for ID anyway. –  TristanK Apr 14 '11 at 21:58
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ID is by far the most common abbreviation.

However, you could argue that Id. is more syntactically correct - a period is often used in abbreviations, particularly when the abbreviation can be confused, as ID could be with Id, the psychoanalytical term.

Nevertheless, I would stick with ID - It's certainly the overwhelming preference in software development, and I think the understanding would be largely ubiquitous.

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Funny that you don't have the one that I think would make the most sense.

Since it is one word, having a period after the first letter doesn't make any sense.

It is not an acronym, so I would rule out ID.

"id" is actually a completely different word that refers to a part of a psyche and "Id" is how you would write it at the beginning of the sentence.

That rules out all for your suggestions. "Id." is what I prefer. "Id.s" being its plural.

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"ID" is hugely common, though, and "Id." is practically never used as far as I can see. –  Neil Coffey Apr 14 '11 at 22:27
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