Continuing this little series on the finer points of quoting stuff in your writing, we turn to the simple but strangely tricky subject of quotation marks.
Quotation marks have several purposes, as you know. They’re used for quoting speech or third-party texts, or for highlighting particular words, denoting the titles of songs, and so on; a very common usage in polemical writing is scare quotes.
So, given all these purposes, when should you use single marks, when should you use double marks, and when and how should you mix them?
The First Rule: Which should you use, single or double?
The first rule of single vs double quotation marks is that there is no first rule. In other, words, it’s up to you which you choose to use. But there are common practices.
The modern convention in British English publishing is to use single quotation marks (‘ ’) because they are regarded as neater and less obtrusive, whereas in American English double quotation marks are preferred (“ ”). Which kind you use is really a matter of preference, but you must be consistent: choose the one you like, and stick with it. Think of this as Rule 1A: as with a lot of textual style issues, consistency is king.
The Second Rule: Mixing single and double quotation marks
There is a fairly widespread misconception that if you use single marks for quoted speech, you should use double marks for other purposes, such as highlighting (or vice versa; double for speech, single for other uses). This is not proper, orthodox usage. Whether you have settled on single or double quotation marks, you should use the same kind for all purposes, including speech, highlighting, titles, and all forms of quotation.1 For example:
‘Have you heard this?’ she asked, and played the introduction to ‘Blue Velvet’. It was the first ‘real’ song he’d heard all day.
The only situation in which single and double quotation marks should appear in the same text is when one instance is nested within another. For example:
‘Have you heard “Blue Velvet”?’ she asked.
I have a theory about the origin of the single/double speech/highlight misconception. I think it may come from a misunderstanding of the commonly heard terms ‘quotation marks’ and ‘inverted commas’. I suspect that this illusory distinction of terms may have misled some people into thinking that they are different things. They aren’t.
There is one arcane exception to the No Mixing rule, a situation in which it is correct to use double quotation marks for quotations and single marks for highlighting. It’s a rare one. In some specialised philosophy and theology texts, double marks are used for standard quotation purposes (as described above), while single marks are used to denote instances when a word is being given an unusual or special meaning.
So, there you have it. Using quotation marks is a lot simpler than you thought it was. Choose the one you like and stick with it!
 Disclaimer: I have come across some small publishers who use their own in-house rules, and do practise mixing of single/double marks in the proscribed manner. With in-house rules, all bets are off; each publisher can do as it likes.