In a time when easy word processing has allowed for typeface styles such as italics to be easily incorporated into text, 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:
As most school students know, titles of pieces of art, writing or communication should always be underlined. Although the underline is currently being phased out by the italic text, many teachers and professors still prefer the underscore.
Titles to underline (or italicize) include:
Where this rule becomes tricky is in the details:
The names of famous vehicles are to be italicized. For example:
On the other hand, airlines, vehicle brand names such as Dodge Charger, and types of vehicles are not underlined.
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 or bon voyage can be underlined in text. (However, etc is not underlined.)
Underline words that are onomatopoetic (stand in for sounds) such as:
These words might look fine on their own, but it doesn't hurt to distinguish them from other words. So, these words are commonly 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. For example:
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 which need to be set apart from the words around them. For example:
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.