In handwritten and typewritten material—where italics are impossible to render—titles of works normally italicized, words used as words, and letters used as letters are underlined:

MLA Handbook

Accommodation has two c’s and two m’s.

In digital environments, if italics and underlining are unavailable and you need to refer to the title of a work normally italicized, insert an underscore on either side of the title:

_MLA Handbook_

For words used as words and letters used as letters, use quotation marks:

Put “i” before “e” except after “c” or when sounding like “a” as in “neighbor” and “weigh.”

But if the quotation marks would cause clutter, as before an apostrophe—

“Accommodation” has two “c”’s and two “m”’s

then revise to avoid the problem:

The letter “c” and the letter “m” appear twice in “accommodation.”