Bold or italic—think of them as mutually exclusive. That is the rule #1.
Rule #2: use bold and italic as little as possible. They are tools for emphasis. But if everything is emphasized, then nothing is emphasized. Also, because bold and italic styles are designed to contrast with regular roman text, they’re somewhat harder to read. Like all caps, bold and italic are fine for short bits of text, but not for long stretches.
Nevertheless, some writers —let’s call them overemphasizers—just can’t get enough bold and italic. If they feel strongly about a point, they won’t hesitate to run the whole paragraph in bold type. Don’t be one of these people. This habit wears down your readers’ retinas and their patience. It also gives you nowhere to go when you need to emphasize a word. That’s no problem for overemphasizers, who resort to underlining bold text or using a lot of bold italic. These are both terrible ideas.
With a serif font, use italic for gentle emphasis, or bold for heavier emphasis.
If you’re using a sans serif font, skip italic and use bold for emphasis. It’s not usually worth italicizing sans serif fonts—unlike serif fonts, which look quite different when italicized, most sans serif italic fonts just have a gentle slant that doesn’t stand out on the page.
Foreign words used in English are sometimes italicized, sometimes not, depending on how common they are. For instance, you would italicize your
bête noireand your Weltanschauung, but neither your croissant nor your résumé. When in doubt, consult a dictionary or usage guide. Don’t forget to type the accented characters correctly.
Characters adjacent to the outside edges of the emphasized text—like punctuation, parentheses, brackets, and braces—do not get the emphatic formatting.
See headings for tips on how to avoid the arms race of overemphasis when working with multiple heading levels.
If you need another option for emphasis, consider all caps or small caps.
Some fonts have both a bold style and a semibold style. You can use either for emphasis. I usually prefer bold to semibold because I like the greater contrast with the roman. But semibold is a little easier to read.
Some fonts have styles that are heavier than bold, like black or ultra. These weights are usually intended for large sizes (for instance, headlines) and don’t work well at body text sizes.