Text can be emphasized in LaTeX using
\textit. For what kinds of text or situations is it recommended to use such emphasis in formal texts? For example, in an article, paper or thesis, when and on what elements should one apply emphasis (or italics)?
closed as off topic by egreg, lockstep, Lev Bishop, Gonzalo Medina, Leo Liu Aug 31 '11 at 12:01
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On the method of creating emphasis in LaTeX: It's better to use
Separately, you ask when and where to use emphasis in "formal" texts. (By "formal texts," I assume you mean texts that don't consist of listings of computer code, say.) By the way, a very good general reference on the topic of the use of emphasis is Robert Bringhurst's book, "The Elements of Typographic Style." Another resource to consider is the Wikipedia page on the uses and methods for emphasis in typography, available at http://en.wikipedia.org/wiki/Emphasis_%28typography%29.
It is certainly widely preferred in good typography to use an italic font (or slanted-roman font, if you prefer) for emphasis within running text rather than to use boldface or, gasp, to underline words. What exactly should be emphasized will of course depend greatly (nearly exclusively?!) on what the formal text is about and what its main points are. The text in the bodies of theorems, corollaries, etc. is often typeset in emphasized mode, presumably both to highlight the importance of the material and to provide a visual offset from the material that comes before and after such environments. In fiction literature, thoughts that a character expresses to him/herself without saying them out loud are very frequently rendered in emphasized mode. Foreign-language words in running text are often set in italics; in this case, it's not so much for emphasis than it's to flag the special form/usage of the words to the readers who might otherwise become confused.
The possibilities for using emphasis to aid your readers' understanding are unbounded, I'm sure.
From Knuth's TeX Book, here is some consideration: