Q. Should the apostrophe in an italicized word in possessive plural form be italicized? Example: If I italicize the possessive form of the word pirates, would the apostrophe also be italicized?
A. That depends. If you’re referring to the plural possessive form of the word pirates as a word, then italicize the whole thing, including the apostrophe: pirates’. But if you’re using italics for emphasis, leave the apostrophe in regular text. For example, “It was the pirates’ ship, not mine, that sank.”
The difference, however, between ’ and ’ will go unnoticed by most readers—even those of us who scrutinize such things for a living—so let’s switch to the singular to confirm our choices. To refer to the possessive pirate’s as a word, you’d put the whole thing in italics (as it is styled in this sentence). But for emphasis—that is, to single out the pirate’s ship as opposed to some other ship—italics are best reserved for pirate alone (as styled in this sentence, between the dashes). Even in the singular, this is an extremely fine distinction that will go unnoticed by many. But it recognizes that the possessive ending can be considered independently of the word to which it attaches, as “belonging to” would be in “the ship belonging to the pirate.” That final period, in case you’re wondering, isn’t in italics.
For italics for emphasis, see CMOS 7.50; for words used as words, see CMOS 7.63.