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.

The “track changes” 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.

“But the Blue­book re­quires un­der­lin­ing.” No, it doesn’t. In its rules for prac­ti­tion­ers, the Blue­book chooses to “keep the tra­di­tion of un­der­scor­ing cer­tain text”, but prac­ti­tion­ers “may sub­sti­tute ital­ics wher­ever un­der­scor­ing is used”. Blue­book at 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 Blue­book so 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.