An underline, also called an underscore,[1] is a more or less horizontal line immediately below a portion of writing. Single and occasionally double ("double-underscore") underlining is used in hand-written or typewritten documents as a way to emphasise key text.[2]

In printed documents underlining is generally avoided, with italics or small caps often used instead, or (especially in headings) 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.


The following kinds of underlines are used on manuscripts to indicate the 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
  • dots under letters to indicate text to be ignored.

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 the 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 text; that is deprecated in favor of the CSS style {text-decoration: underline}. The elements may also exist in other markup languages, such as MediaWiki. The Text Encoding Initiative (TEI) provides an extensive selection of related elements for marking editorial activity (insertion, deletion, correction, addition, etc.).


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.[3]

  • single underline: a̲b̲c̲d̲e̲f̲g̲h̲i̲j̲k̲l̲m̲n̲o̲p̲q̲r̲s̲t̲u̲v̲w̲x̲y̲z̲0̲1̲2̲3̲4̲5̲6̲7̲8̲9̲
  • double underline: a̲̲b̲̲c̲̲d̲̲e̲̲f̲̲g̲̲h̲̲i̲̲j̲̲k̲̲l̲̲m̲̲n̲̲o̲̲p̲̲q̲̲r̲̲s̲̲t̲̲u̲̲v̲̲w̲̲x̲̲y̲̲z̲̲0̲̲1̲̲2̲̲3̲̲4̲̲5̲̲6̲̲7̲̲8̲̲9̲̲

"Simulated" underlines in plaintext

In plain-text (ASCII) computer 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 (typically red or wavy or both) 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 (simplified Chinese: 专名号; traditional Chinese: 專名號; pinyin: zhuānmínghào; literally "proper name mark", used for personal and geographic 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 (simplified Chinese: 书名号; traditional Chinese: 書名號; pinyin: shūmínghào; literally, " 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.

See also


  1. The American Heritage Dictionary of the English Language
  2. Butterick, Matthew. "Underlining: absolutely not". Practical Typography. Retrieved 5 August 2015.
  3. "6.2 General Punctuation" (PDF). The Unicode Standard. Version 11.0.0. Mountain View, CA: The Unicode Consortium. 2018. p. 273. ISBN 978-1-936213-19-1. Retrieved 2018-12-12. Spacing Overscores and Underscores. U+203E OVERLINE is the above-the-line counterpart to U+005F low line. It is a spacing character, not to be confused with U+0305 COMBINING OVERLINE. As with all overscores and underscores, a sequence of these characters should connect in an unbroken line. The overscoring characters also must be distinguished from U+0304 COMBINING MACRON, which does not connect horizontally in this way.
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