Underlining inside your text can be useful for emphasis when you write longhand.
It was useful when your ancestors typed on a typewriter. “Underlining is an unsightly relic from the typewriter era, when italics weren’t usually available.” Bryan A. Garner, ‘Ban Underlining in your legal documents,’ LawProse Lesson 271, lawprose.org (accessed 17 Jan. 2020).
With a typewriter, our ancestors used underlining to simulate italics. Italics are “the print equivalent of underlining.” Lynne Truss, Eats, Shoots & Leaves: The Zero Tolerance Approach to Punctuation, Gotham Books, 2004, 146.
Now you can italicize to your heart’s content on your computer, device, or tablet. Now, you emphasize, when you must, with italics and (rarely) with boldface.
Avoid using underlining for emphasis, for instance to highlight your prayer or relief in an application or other court document, or to set out the questions for determination in a brief, or to set out a corporate resolution.
Underlined text is not as easy to read as free text. Underlining can obscure, if not obliterate, the descenders of these lowercase letters: f, g, j, p, q, and y. Underlining also obscures commas and semicolons. Underlined text is unkind to the human eye. “Underlining is ugly, both on-screen and in printouts.” Garner, The Redbook, 4th ed., 85.
For headings, prefer boldface to underlining. Use boldface for your letters’ subject lines. Don’t combine boldface and underlining in your headings or anywhere else: that’s the stylistic equivalent of wearing a belt and suspenders.
Here’s another reason you shouldn’t underline: your work may end up on the Internet. On webpages, underlining signifies a hyperlink. Your underlined text will create confusion unless the word or phrase you underline is actually a hyperlink. Readers who click on your presumed hyperlinks will be frustrated.