Grammar Rules for Underlining

Similar to italicizing, underlining is used to emphasize certain words or phrases. Nothing makes a document seem more unorganized and unprofessional as grammatical errors. Most people learn the rules of grammar in school, but in the age of technology and word processing it's easy to forget certain rules and simply rely on a spell-checker to polish your work. Rules for underlining can sometimes be used incorrectly, but applying these rules can insert the right emphasis in your written words and make your sentences more precise and coherent.
A common reason to use underlining is to emphasize certain words in the sentence. It is acceptable to underline a piece of writing that you feel is important in defining the sentence. An example would be: "Do not leave your dirty boots on the carpet." The word "not" could be underlined here because it is the emphasis of the sentence. Another example of underlining for emphasis occurs when a specific word in a sentence is meant to accuse someone. The word "you" in this sentence could be underlined for emphasis: "You are the murderer!"
Though italics are more often used for titles, it is still acceptable to underline titles. Examples of titles that could be underlined include magazines, books, operas, plays and epic poems. Titles or artistic pieces can also be underlined. Titles of movies and television shows are underlined, but not individual television episodes. Fine art, such as titles of paintings and sculptures, can also be underlined.
The rules for underlining names can be confusing. Names of ships can be underlined, but not the designations such as "USS" or "HMS." Names for other modes of transportation such as trains, airplanes and space shuttles should always be underlined. Neither vehicle names nor names of individual people should be underlined. An exception to this rule involves land vehicles. While brand names should not be underlined, it is acceptable to underline vehicles that are known by a specific name. Examples of this would be Greased Lightning from the movie "Grease" or Scooby Doo's Mystery Machine.
Some words are underlined to show their difference between them and the common English words around them in the sentence. Words such as bon voyage or gesundheit can be underlined. It is acceptable to underline et cetera when it is spelled out, but never underline the abbreviation etc. Onomatopoetic words, or words that represent sounds, can also be underlined. Examples of these words include grr, bzzz, boom, or kerplunk. Although many people choose to italicize these words, it is also acceptable to underline them as well.
Another example of underlining occurs in words that are referred to as words. This happens in sentences such as: "Anyone can spell the word word." In that sentence, the second use of "word" could be underlined. Numbers and letters could also be underlined if they are referred to in a sentence, such as: "Count all the 5s and the Cs in the survey results." In that sentence, you could underline "5s" and "Cs." Another reference word you can underline is a word that is used in a definition. For example, in the sentence "Apathy is defined as the absence or suppression of emotion or excitement," the word "apathy" could be underlined to show that it is the word being defined.

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