Often when we’re writing nonfiction we need to refer to words in such a way that the term being used is itself the subject of the discussion, rather than the concept the term describes. If I say “My Sunday school students have difficulty understanding the concept of propitiation,” it means something very different from “English is her second language, so she has difficulty understanding the word propitiation.”
When in your writing you need to discuss the word or phrase itself rather than the concept described by the term, put the term in italics. I have seen writers try things like this:
In this study, we’ll discuss the etymology of “decimate,” “enormity,” and “nauseous.”
When terms in quotation marks like this are scattered across a page, the marks soon become distracting. Using italics instead makes for a cleaner presentation:
In this study, we’ll discuss the etymology of decimate, enormity, and nauseous.
There are some cases where you might want to use quotation marks. The example given by Chicago is when you are discussing both Spanish and English terms at the same time. Foreign words are italicized, so it might confuse the reader to italicize the English terms as well. So at 7.58, Chicago gives this example:
The Spanish verbs ser and estar are both rendered by “to be.”
What About Letters?
The treatment of letters as letters is similar:
When he leaves work for the day, he puts a red X on the calendar.
Note that Chicago says such letters are “usually italicized.” They are not in expressions like U-turn, L-shaped desk, or T-shirt, where the letter is used to represent the shape of the thing rather than the letter’s own linguistic properties.
Roman type, however, is traditionally used in two common expressions…
Mind your p’s and q’s!
dotting the i’s and crossing the t’s
—Chicago Manual of Style, 16th edition, 7.59
When referring to letter grades in school, use neither italics nor an apostrophe:
She got all As this semester.
I know it looks weird. Trust me on this one. Do not stick an apostrophe in there. In context, no one is going to mistake it for the word as. The only time you need to use an apostrophe with a plural of a letter is if it is lower case:
The word llama has two l’s and two a’s.
Note that the letters l and a are italicized, but the apostrophe and the s are not.
During this discussion of Mechanics, I keep quoting from The Chicago Manual of Style, don’t I? That’s because it’s a 1,000-page book, so I can quote without exceeding fair use. Yet I said you didn’t need to buy a copy. I meant that. Do, however, pick up a copy of Proofreading Secrets of Best-Selling Authors