In a printed document, don’t underline. Ever. It’s ugly and it makes text harder to read. See for yourself—
Underlining is another typewriter habit. Typewriters had no bold or italic styling. So the only way to emphasize text was to back up the carriage and put underscores beneath the text. It was a workaround for shortcomings in typewriter technology.
Your word processor doesn’t suffer from these shortcomings. If you feel the urge to underline, use bold or italic instead. In special situations, like headings, you can also consider using all caps, small caps, or changing the point size.
Not convinced? I invite you to find a book, newspaper, or magazine that underlines text. Aside from supermarket tabloids—was that the look you were going for?—you won’t find any.
Another reason underlining looks worse than bold or italic: underlining is mechanically applied by the word processor. Bold and italic styles are specially designed to match the regular style of the font.
“trackchanges” feature of your word processor will underline text added to the document. This is fine. In fact, it’s one more reason not to use underlining for emphasis— so that it won’t be confused with revision marks. “Butthe Bluebookrequires underlining.” No, it doesn’t. In its rules for practitioners, the Bluebookchooses to “keepthe tradition of underscoring certain text”, but practitioners “maysubstitute italics wherever underscoring is used”. Bluebookat 3 (20th ed. 2015). In later rules, whereas the 19th edition said to “underscore(or italicize)”, the 20th edition says to “italicize(or underscore)”. (For instance, Bluebook rule B2.) These are the details that make each new edition of the Bluebookso compelling.
On a website, it’s idiomatic to underline bits of clickable text (also known as
hyperlinks). But don’t underline other text—visitors will be confused when their clicking goes unanswered.