In a time when easy word processing has allowed for typeface styles to run amok, the importance of underlined (or underscored) text has been greatly diminished. Underlined text is often used in the middle of sentences indiscriminately as a way of providing emphasis, sometimes in superfluous addition to italics and boldface. Even when a discerning individual attempts to use the underscore correctly, he or she often ends up applying the attribute to a word or phrase that does not require an underline.
In reality, the rules of underlining words and phrases are very few. Similar to italicization and bolding, underscoring text is used primarily to draw emphasis, to set proper titles apart from regular text, to connote a foreign word, and to reference letters and numerals out of context.
As most school students know, titles should always be underlined. Although the underline is currently being phased out by the italic text, many teachers and professors still prefer the underscore.
Book titles are always to be underlined, without exception. Many other pieces of writing follow suit. Magazine titles, plays, operas, long (especially epic) poems, even longer songs are to be underlined when referenced in a piece of writing. Artistic works of different types may be underlined as well. Films and television shows are underlined, as are pieces of art and sculpture, pamphlets, and even speeches may be underlined. Where this rule becomes tricky is in the details: episodes of television shows, short stories, and religious works are not underscored. Some newspapers refuse to underline the “the” in their names, or even the city of origin. This depends entirely upon the preference of the paper.
Certain names should be underlined. Of course, the names of people, however famous and prominent, are never underlined. The names of vehicles, when they are famous, are to be italicized. For example, the names of ships (with the exception of the designation HMS, RMS, or USS) are always underlined. The same goes for space shuttles, trains, and famous airplanes. On the other hand, airlines, vehicle brand names such as Dodge Charger, and types of vehicles are not underlined. Even the names of land vehicles should be underlined when the land vehicle is known by a specific name. Although it is not commonly practiced, it is entirely appropriate to underline Mystery Machine, Weinermobile, and Greased Lightning.
Unfamiliar and foreign words are often underlined to distinguish them from the English language text that surrounds them. Even commonly used words such as et cetera, bon voyage, or gesundheit can be underlined in text. (However, etc or &c are not underlined.) If a word is unusual, or on loan from another language, it may be underlined. Underline words that are onomatopoetic, or words that stand in for sounds. Kerrow, kerplunk, grr, and bzzzz might look fine on their own, but it doesn’t hurt to distinguish them from other words. More commonly, these words are italicized in this day and age.
The need to emphasize the word is among the most common reasons for underlining. Other type styles, such as boldface and italic, are used as well. If a particular word or phrase in a piece of text is to be emphasized, particularly when the piece of text is to be read aloud, underline it. If you’re not sure, underline it just in case. Fight the urge in these instances to type the word in all caps, which is not a commonly accepted way to demonstrate emphasis. Emphasis may be added to a word to make it seem accusatory in nature, as in “you’re the real problem here, buster!” You may also use it to underscore something in a piece of writing that is particularly important: “books are not to be placed on the floor.” The underline gives the word a bit more gravity.
Underline words referred to as words (the word word), numbers or letters referred to out of context (the 1’s and the a’s), and in definitions (stupidity is defined as…). If you’ve mastered all of these, you’ll be underlining exactly as the underline is intended to be used—no more and no less.