In a printed document, don’t underline. Ever. It’s ugly and it makes text harder to read. See for yourself—
Underlining is another dreary typewriter habit. Typewriters had no bold or italic styling. So the only way to emphasize text was to back up the carriage and type underscores beneath the text. It was a workaround for shortcomings in typewriter technology.
Neither your word processor nor your web browser suffers 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. That look is mostly associated with supermarket tabloids. If that’s the impression you want to make with your writing, by all means, use underlining. If not, don’t.
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 basic roman style of the font.
The “track changes” feature of your word processor will underline text added to the document. This is fine. In fact, it’s another reason not to use underlining for emphasis—so readers don’t confuse text that’s marked as a revision with text that happens to be underlined.
On the web, hyperlinks have traditionally been underlined. But email lost its hyphen, and the internet lost its capitalization. Consistent with those signs of maturity, I think it’s time to move beyond underlined hyperlinks too.
New York Times, New York magazine, the Washington Post, Bloomberg, Google, Amazon, Apple, GitHub, and Wikipedia. Even eBay, paragon of ’90s website design, has relented. Is underlining links dead? Maybe not quite. Dying? For sure. “Oh come on! Everyone underlines links!” Some who don’t: the