An underline, also called an underscore, is a more or less horizontal line immediately below a portion of writing. Single and occasionally double underlining is used in hand-written or typewritten documents as a way to emphasise key text. In printed documents underlining is generally avoided, with italics or small caps often used instead, or using capitalization or bold type. In a manuscript to be typeset, various forms of underlining were therefore conventionally used to indicate that text should be set in special type such as italics, part of a procedure known as markup. Underlines are sometimes used as a diacritic, to indicate that a letter has a different pronunciation from its non-underlined form.
Manuscripts
The following kinds of underlines are used on manuscripts to indicate the special typefaces to be used:
dots under letters to indicate text to be ignored.
Use with computers.
In web browsers, default settings typically distinguish hyperlinks by underlining them, but both users and websites can change the settings to make some or all hyperlinks appear differently.
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.
single underline: a̲b̲c̲d̲e̲f̲g̲h̲i̲j̲k̲l̲m̲n̲o̲p̲q̲r̲s̲t̲u̲v̲w̲x̲y̲z̲0̲1̲2̲3̲4̲5̲6̲7̲8̲9̲
In plain-textcomputer files, including plain-text e-mails, if 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".
As a marker for incorrectness
Underline is often used by spell checkers to denote misspelled or otherwise incorrect text.
Underlines in non-Latin scripts
In Chinese, the underline is a punctuation mark for proper names. Its meaning is somewhat akin to capitalization in English and should never be used for emphasis even if the influence of English computing makes the latter sometimes occur. A wavy underline 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.
