Underlines are sometimes used as a diacritic, to indicate that a letter has a different pronunciation to its non-underlined form.
[edit] Use with computers
In web browsers, default settings typically distinguish hyperlinks by underlining them (and usually changing their color), but both users and websites can change these settings to make some or all hyperlinks appear differently (or even without distinction from normal text).
The HTML special inline element <ins>, denoting inserted text, is often presented as underlined text. HTML also has a presentational element <u>, denoting underlined textthis is deprecated in favour of the CSS style {text-decoration: underline}. These elements may also exist in other markup languages, such as MediaWiki.
In plain-text (ASCII) computer files, including e-mail communication, where underlining is not possible, it is often indicated by surrounding words with underscore characters. For example, "You must use an _emulsion_ paint on the ceiling".
Unicode has the combining diacritic "combining low line" at U+0332 ̲ that results in an underline when run together: u̲n̲d̲e̲r̲l̲i̲n̲e̲. Not to be confused is the combining macron below.
Underline (typically red and/or wavy) is often used by spell checkers (and grammar checkers) to denote misspelled or otherwise incorrect text.
Underlines in non-Latin scripts
In Chinese, the underline is a punctuation mark for proper names (专名号, 专名号pinyin: zhuānmínghàoliterally “proper name mark”, used for personal and geographic names). Its meaning is somewhat akin to capitalization in English and should never be used for emphasishowever, due to the influence of English computing, the underline is now sometimes used for emphasis. A wavy underline (书名号, 书名号pinyin: shūmínghàoliterally “book title mark”) serves a similar function, but marks names of literary works instead of proper names.
In the case of two or more adjacent proper names, each individual proper name is separately underlined, so there should be a slight gap between the underlining of each proper name.
Manuscripts
The following kinds of underlines are used on manuscripts to indicate special typefaces to be used.
single underline for italic type
double underline for small caps
triple underline for full capital letters (used among small caps or to change text already typed as lower case)
wavy underline for boldface
en.wikipedia.org/wiki/Underline
When writing about other works, it's hard to decide when to underline (or place in italics) a title and when to place it in double quotations. Note that some publications have a "house style" that must be followed. When in doubt, however, these guidelines from the Modern Language Association may help:
For titles of written or musical works that are published within other works use double quotationsunderline or italicize names of works published by themselves:
ex. I just read the short story "Looking for Jake" in China Miéville's anthology of the same name, Looking for Jake.
ex. Beckett's play Waiting for Godot will be performed next season.
ex. Devo's second album, Duty Now for the Future, has one of my favorite songs, "Swelling Itching Brain."
ex. Yes, I went to a science-fiction convention. I really enjoy the original Star Trek TV series, especially the episode "Return of the Archons," and the first three Star Wars films, especially The Empire Strikes Back, okay?
ex. I read the story "All about the Bronx" in the city section of today's New York Times.
ex. I have subscribed to my favorite magazine, The Atlantic, for many years.
For names of artwork, always use italics or underlining:
ex. We have a copy of Edward Hopper's painting Nighthawks in the Writing Center lobby. I always think about it when I'm listening to Tom Wait's CD Nighthawks at the Diner.
For the names of famous aircraft, ships, and spacecraft, always use italics or underlining:
ex. I built scale models of the USS Nimitz and the space shuttle Discovery last year.
http://writing2.richmond.edu/writing/wweb/titles.html
