Originally, the ‘Italic hand’ was used to describe the slightly angled, elegant lettering of Italian medieval manuscripts. As such, it was a font in and of itself. Today, while there are exclusively italic fonts, it is much more commonly seen as a font variant. As saavikam77 asks, though, how should one use italics?
The most common way to use italic text is to stress or emphasize specific words, and there are several different possibilities for this emphasis.
Scientific nomenclature. When it comes to emphasis with italics, the only specifically defined usage is that biological names in the Linnaean system must be italicised, as must symbols for physical quantities. For example:
“Really, Jethro, all I can tell you before completing my autopsy is that the body is of the species Homo sapiens.”It's quite likely that you'll never need to write a physical quantity into a story because these aren't units of measure like kg or lb, but are typically single letters which represent the result of measurement: for example c0 is used to describe the speed of light in a vacuum. Rather than try and explain in detail the specifics of using italics in scientific texts, it's easiest to point at a paper published by the International Union of Pure and Applied Chemistry which can be read as a PDF here.
Foreign words. By convention, foreign words are often put in italics. However, many words like 'de facto', 'detour', and 'etc.' have become part of common English usage, so should not be italicised. Truly foreign words, though, should be emphasised with italics -- for example, someone writing about foreign foods they had experienced.
Abby looked with rapture at the huge slice of Schwarzwälder Kirschtorte McGee had bought her by way of an apology.How do you know if you should italicise a foreign word? If you can find the word in an English dictionary, you don't need italics.
Titles and names. Italicising the names of artistic works, for example a television show, book, or piece of music, is normally done -- though the actual usage may vary depending on the house style. For example, newspapers will tend to avoid italics in nearly every situation because it can be very difficult to read in newsprint. Names of ships and houses are typically included in that usage – though simply capitalizing the name would be sufficient, again depending on the house style. For example:
DiNozzo watched as the view of the USS Seahawk rapidly fell away behind the COD.Even if italics are not used the name of the ship is clear, so it is personal preference as to whether italics are required. An example of a artistic work follows:
Whenever he'd experienced one of the most horrifying cases, Aaron Hotchner always found himself drawn to the "Dies Irae" from Mozart's Requiem.Showing logical emphasis. The sentence ‘I hope you can help me’ can be read six different ways by putting the emphasis on a different word in the sentence in turn. A simple way of demonstrating intention is to use italics, e.g. ‘I hope you can help me.’
Words, letters, or numbers used as themselves. It's also important to express emphasis with italics when you are using a word, number, or letter as itself. For example:
"I'm Special Agent Gibbs. That's spelled with two b's."or
"Mr. Palmer, the origin of the word renal is quite interesting, in that it is based on the Latin renos, as I am sure you are aware."So where should italics be used outside these examples? For example, pressing the ‘up’ button in an elevator? While this could be emphasised using italics, it makes as much sense to simply use quotes as above – or better yet make sure the sentence is structured so that neither is required.
As soon as the elevator had started its descent, Gibbs flicked the emergency stop switch, bringing it to a sudden halt.The same goes for nicknames – it makes more sense to put them in quotes than it does to emphasise them with italics, but neither is actually required. The same can be true when describing the text on a sign. Depending on how that text is being referenced, emphasis may or may not be required. One situation in which it does make sense is as follows:
“Here we go, McGenius. You can tell it’s the right place because of the sign saying ‘murderer lives here’ on the front lawn.”As you can see, the nickname works fine without emphasis, and the text of the imaginary sign is highlighted using quotes, which provides a better emphasis than italics would have done.
Italics can also be used in other situations: the other side of a phone conversation, a character’s thoughts, or the contents of a letter they may be reading. The bottom line is that no matter how you end up deciding to provide emphasis using italics, you must be consistent in that usage. Don’t use italics one way in one place, and a different way elsewhere - it will just confuse your readers.