The 16th edition of the Chicago Manual of Style has twelve individual index entries under italics, and one of those items, “uses, other” has twenty-one sub-entries. So there are a lot of rules, but let’s look at a few that I see come up in fantasy and science fiction novels and stories. I’ll leave a few of the more obscure bits for you to find on your own if you’re writing something more technical.
First of all, in their handy glossary, the CMS defines italic as:
A slanted type style suggestive of cursive writing (like this). Contrast roman.
And roman as:
The primary type style (like this), as distinguished from italics (like this).
So those are our basic terms. And then a quick bit of advice on how to access italics: It’s always better to highlight a word and choose italics rather than to identify it as a whole new font, or worse, a new style. There is no reason in 2016 that you should underline something to indicate italics. Just highlight it, click that I icon or command-i (in MS Word, at least, and command might be Mac only, I don’t know about those crusty old PC things) and you’re good to go.
Actually making text italic is easy, knowing when and why to do that can be a bit more complicated. Chances are you’ll use italics in one of four categories: emphasis, titles, foreign words, and thought/alternative dialog. We’ll look at the basic rules from the CMS for as many as we can find, but we might have to dip into other sources.
This is when, especially in dialog, you want to make sure that a particular word or short set of words really lands, making sure that this word or these words are seen as particularly important. Here’s a good warning, though, from the CMS, that I hope you’ll take to heart:
Overused, italics quickly lose their force. Seldom should as much as a sentence be italicized for emphasis, and never a whole passage.
Please take this to mean that if, say, you are including the text of a letter in your manuscript, that text should not be set entirely in italics. If you have little interstitial scenes or some other device like journal entries or something like that, also, do not set them all in italics. Italics were not meant to be read for sentence after sentence, page after page, and doing so can be really hard on the eye. This is another instance where your readers might not be able to articulate that. They might not say, “I hated all the italics—they hurt my eyes.” But they will hate all the italics, because they do hurt their eyes.
I’ll add to this that italics, and italics only, are to be used for emphasis, never all caps. If someone is really screaming really loud! the exclamation mark and the surrounding context will have to carry that. And please resist any temptation to MIX these Elements all UP or to use any more than one Punctuation Mark at the end of the sentence!?!…!
Remember: You’re writing a story, not some kind of concrete word art.
Set the titles of the following things in italics: books, journals, movies, and paintings. (8.2) Also “the names of ships and other craft, species names, and legal cases.”
For fantasy and science fiction authors, this means that “in world” book titles like The Book of Common Spells by Galen Wizardson or General Principals of Anti-Gravity by Bert Einstein, Jr. would be set in italics.
We’ll also have ships, including starships and future or fantastical versions of ships (steampunk airships, etc.), so make sure you’re italicizing Enterprise or Sea Sprite. But in a separate rule (7.28) the possessive s should be set in roman:
I was the last of the Enterprise’s redshirts to get the safety briefing.
Going back to the rule for titles (8.2) note that sub-titles or chapter titles should be set in quotation marks, as should the titles of poems. The title of a series should be set in roman, without italics. So you would write: The Legend of Drizzt, Book I: Homeland, where the series title is the Legend of Drizzt and the title of the individual book is Homeland.
That mention of species is worth noting in that what they mean there is, homo sapiens, not human. So if you have elves or unicorns or halflings in your story, elf, unicorn, halfling, etc., should not be italicized, but if you invent some kind of Latin(ish) classification for unicorns, equus magickus, or something, okay then.
Note, please, that the names of businesses like inns, taverns, restaurants, etc. are not set in italics:
While waiting for the rest of Merry Widow’s crew to unload the coffins, Galen sat at the bar in the Weeping Pony Inn and read the last few chapters of the Blood Red Steel Saga, Book XXXVI: Bronwyn’s Lament with a tear in his eye.
Going back to that idea that you don’t want to invent any rules of grammar, style, and usage you don’t have to, foreign words will include words in any language you create on your own, so what’s good for French is good for Dwarvish. From the CMS:
[7.49] Italics are used for isolated words and phrases in a foreign language if they are likely to be unfamiliar to readers. If a foreign word becomes familiar through repeated use throughout a work, it need be italicized only on its first occurrence. If it appears only rarely, however, italics may be retained.
I’ve used the example of the Forgotten Realms piwafwi—the peculiar magical cloak the drow wear—as an example of an invented “foreign” word you can’t “translate” into English. There is no real world analog for a piwafwi, so that’s what we have to keep calling it.
Following this rule to the letter, in a Forgotten Realms book, anyway, we’d see only the first instance of piwafwi in italics and if it’s referred to again in the text it would be set in roman.
Here’s where I feel I have to break from the CMS, though they do give me an out with the last sentence of the above quote. I tend to leave foreign words like this in italics throughout, adhering, I’ll admit, perhaps a bit too closely to the escape hatch provided. Since the CMS seems reluctant to define in concrete terms what constitutes “only rarely,” I’m going to have to. And in section 11.95 of the CMS, I’m given another out: “If used throughout a work, a transliterated term may be italicized on first appearance and then set in roman.” That says to me: “or, you can leave it italicized . . .” at least as much as it says, “You don’t have to italicize it in the first place.”
The style at TSR/Wizards of the Coast was always that a character’s thoughts were left in roman, and italics were reserved for magical or telepathic communication. If your story contains some form of magical or telepathic communication, or some other alternative means of communication (hand signals, flashing lights, etc.) I think you’d really benefit from adopting that style.
The CMS seems to kind of punt on this subject:
[13.41] Unspoken discourse. Thought, imagined dialogue, and other interior discourse may be enclosed in quotation marks or not, according to the context or the writer’s preference.
I honestly can’t think of a single book in which I’ve seen thoughts in quotes—how can we tell that apart from spoken dialog?
Thoughts. Unspoken thoughts, which might appear in context with dialogue, are often italicized rather than set roman with quotation marks.
Even there, it’s not stated in absolute terms.
So on this score, taking into account whatever special needs your story may require, set your own standards, be as careful as you can be to adhere to those standards yourself, then communicate those standards to your editor(s).
As with all rules in creative writing these are all made to be broken, but I’ll remind you again of the significant difference between breaking a rule on purpose for some specific effect, and not knowing the rule in the first place.