Underscore

An underscore or underline is a line drawn under a segment of text.^[a] In proofreading, underscoring is a convention that says "set this text in italic type", traditionally used on manuscript or typescript as an instruction to the printer. Its use to add emphasis in modern finished documents is generally avoided.^[1]

The (freestanding) underscore character, _, also called a low line, or low dash, originally appeared on the typewriter so that underscores could be typed. To produce an underscored word, the word was typed, the typewriter carriage was moved back to the beginning of the word, and the word was overtyped with the underscore character.

In modern usage, underscoring is achieved with a markup language, with the Unicode combining low line or as a standard facility of word processing software. The free-standing underscore character is used to indicate word boundaries in situations where spaces are not allowed, such as in computer filenames, email addresses, and in Internet URLs, for example Mr_John_Smith.

Diacritic

The combining diacritic, U+0331 ◌̱ COMBINING MACRON BELOW, is similar to the combining low line but is shorter. The difference between "macron below" and "low line" is that the latter results in an unbroken underline when it is run together: compare a̱ḇc̱ and a̲b̲c̲ (only the latter should look like abc).^[3][b]

Modern use

In a manuscript (or typescript) to be typeset, various forms of underlining (see below) were therefore conventionally used to indicate that text should be set in special type such as italics, part of a procedure known as markup. In printed documents underlining is generally avoided, with italics or small caps often used instead, or (especially in headings) using capitalization, bold type or greater body height (font size).^[1] Underlining may still be seen in display work.^[4]

A series of underscores (like __________ ) may be used to reserve a blank space in text that is later to be filled in by hand, such as on a paper form. It is also sometimes used to create a horizontal line; other symbols with similar glyphs, such as hyphens and dashes, are also used for this purpose.

In German, Slovene and some other Slavic languages, the underscore has recently gained prominence as the punctuation to form gender-neutral suffixes in gendered nouns and other parts of the speech.^[5]

The underscore is also used in modern editions of Spanish vocal sheet music to indicate elision, instead of the breve below (U+032E ◌̮ COMBINING BREVE BELOW), which is less convenient to input on a computer.

Mathematics

In mathematical notations, underscores are sometimes used in the following contexts:

• vectors. Underscores are sometimes used instead of bold face or a variable with an overarrow. E.g.: ${\displaystyle {\underline {v}}=\mathbf {v} =\left[v_{1}\;v_{2}\right]^{\mathbf {T} }.}$^[6]
• complex numbers. Similar to vectors, underscored variable names can indicate a complex-valued variable, especially if the real-valued variable name is reused to indicate the complex variable. For instance, in optics, ${\displaystyle n}$ is often used for a medium's index of refraction. When the refractive index has a wavelength-dependent extinction coefficient ${\displaystyle \kappa }$, the combined complex index of refraction is sometimes notated ${\displaystyle {\underline {n}}=n+\mathrm {i} \kappa .}$
• minimum bound. Example: ${\displaystyle {\underline {x}}<x<{\overline {x}},}$ where ${\displaystyle {\underline {x}}}$ is the lower bound of ${\displaystyle x,}$ and ${\displaystyle {\overline {x}}}$ is the upper bound.^[7]

Usage in computing

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).^[1]

Manuscripts

Depending on local conventions, the following kinds of underlines may be used inline on manuscripts to indicate the special typefaces to be used:^[17][18]

• single dashed underline for stet, 'let it stand', proof-reading mark cancelled.
• single straight underline for italic type
• single wavy underline for bold type
• double straight underline for small caps
• double underline of one straight line and one wavy line for bold italic
• triple underline for FULL CAPITAL LETTERS (used among small caps or to change text already typed as lower case).

Underlines in Chinese

In Chinese, the underline is a little-used 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.^[19]

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

• Overline
• Space Character
• Strikethrough
• Undertie
• Visible space

Notes

References

External links

Look up underscore, underline, or Appendix:Underline in Wiktionary, the free dictionary.

• "Ngram Viewer: comparison of usage of 'underscore' and 'underline'". books.google.com. Retrieved 9 October 2020.