Use of Single Quotation Marks

The most common reason to use single quotation marks is to quote someone who is quoting someone else. The rules differ in British English, but in American English, you enclose the primary speaker's comments in double quotation marks, and then you enclose the thing they are quoting in single quotation marks. You nest them, with the double quotation marks on the outside and the single quotation marks on the inside. For example, imagine you've interviewed Aardvark for a magazine article about his harrowing ordeal with an arrow, and he said, “Squiggly saved my life when he yelled, 'Watch out, Aardvark.' ”If you're ever in the extremely rare position of having to nest another quotation inside a sentence like that, you would use double quotation marks again for the third nested quote. 

The Associated Press uses single quotation marks for quotations in headlines. It's the convention in certain disciplines such as philosophy, theology, and linguistics to highlight words with special meaning by using single quotation marks instead of double quotation marks. 

It can be hard to see a single quotation mark that’s immediately followed by a double quotation marks, so typesetters sometimes insert something called a thin space between the two. A thin space is just what is sound like: a space that’s thinner than a regular space. "Quote" Versus "Quotation."

Also, a frequent point of confusion is the difference between the words “quote” and “quotation.”  “Quote” is a verb that means to repeat what someone else has said or written. For example, "Aardvark quoted Squiggly." “Quotation” is a noun used to describe what you are quoting, as in "Squiggly's quotation was inspiring." It's common to hear people use the noun “quote” as a shortened form of “quotation,” as in "I filled my notebook with quotes from The Daily Show," but that is technically wrong. It should be, "I filled my notebook with quotations from The Daily Show." I agree the correct way sounds a bit pretentious, and given that a lot of reference sources have extra entries discussing how the misuse is widespread, you aren't going to sound illiterate if you use “quote” incorrectly, but it is still good to know the difference. For more about quotation marks, be sure to visit The Grammar Girl.

[This is an excerpt from a Grammar Girl article that was originally published in 2006.]