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What is the semantic difference between apostrophe and single quote?

I see people use both of them interchangeably, but people never create two words to denote one concept. There should be a difference.

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drm's answer is correct. Nonetheless, I have to take issue with your statement that "people never create two words to denote one concept". In fact, people do this all the time. They're called "synonyms" and we have tons of them. – JSBձոգչ Jul 29 '11 at 13:37
No @JSBangs, I don't think so. Synonyms are fundamentally interchangeable words, not exact words. – Saeed Neamati Jul 29 '11 at 13:42
@JSBangs makes a good point. Think of all the terms we have for the act of coitus, for example. (I chose that one because there are so many examples.) – Robusto Jul 29 '11 at 13:46
@Robusto: That's slightly disingenuous. Exact synonyms abound in slang, obviously, and yours is one of the most common slang referents. But there are few (if any?) exact synonyms in "standard" vocabulary. – FumbleFingers Jul 29 '11 at 14:38
up vote 12 down vote accepted

An easy way to differentiate:

An apostrophe is only used within or at the very end of a word - it is part of the word.

In English, it serves three purposes:

  • The marking of the omission of one or more letters (as in the contraction of do not to don't).
  • The marking of possessive case (as in the cat's whiskers).
  • The marking as plural of written items that are not words established in English orthography (as in P's and Q's, the late 1950's). (This is considered incorrect by some; see Use in forming certain plurals. The use of the apostrophe to form plurals of proper words, as in apple's, banana's, etc., is universally considered incorrect.)

Single quotes are only used around words - they come in pairs, and are not part of any word.

Single or double quotation marks denote either speech or a quotation.

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Some linguists go so far as to say that the apostrophe is a letter. – Mr. Shiny and New 安宇 Jul 29 '11 at 13:27
+1 for thoroughness and balance over the third purpose. Technically I should be upvoting Wikipedia maybe, but it was you who found and linked it in here. – FumbleFingers Jul 29 '11 at 14:33
I edited the Wikipedia quotes to be more true to the original formatting. Hope it helps! – Daniel Jul 29 '11 at 17:46
There seems to be a bit of dispute as to when the third rule should apply. IMHO, it should be applied fairly broadly in cases where it may assist visual parsing and would not introduce ambiguity. For example, when talking about automatic teller machines, the plural is not pronounced "ai-tee-em-ess", but "ai-tee-ems"; to my mind, writing "ATM's" makes clear that the "s" should be regarded differently from the preceding letters. Other typographical indicators (e.g. italics or hairspaces) may be better than an apostrophe, but in many contexts the latter is more likely to copy/paste correctly. – supercat Oct 15 '12 at 22:31

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