Underscore

An underscore or underline is a line drawn under a segment of text. 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. 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).

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). Underlining may still be seen in display work. 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. 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.: v _ = v = [ v 1 v 2 ] T . {\displaystyle {\underline {v}}=\mathbf {v} =\left[v_{1}\;v_{2}\right]^{\mathbf {T} }.}

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, n {\displaystyle n} is often used for a medium's index of refraction. When the refractive index has a wavelength-dependent extinction coefficient κ {\displaystyle \kappa } , the combined complex index of refraction is sometimes notated n _ = n + i κ . {\displaystyle {\underline {n}}=n+\mathrm {i} \kappa .}

minimum bound. Example: x _ < x < x ¯ , {\displaystyle {\underline {x}}<x<{\overline {x}},} where x _ {\displaystyle {\underline {x}}} is the lower bound of x , {\displaystyle x,} and x ¯ {\displaystyle {\overline {x}}} is the upper bound.

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).

History As early output devices (Teleprinters, CRTs and line printers) could not produce more than one character at a location, it was not possible to underscore text, so early encodings such as ITA2 and the first versions of ASCII had no underscore. IBM's EBCDIC character-coding system, introduced in 1964, added the underscore, which IBM referred to as the "break character". IBM's report on NPL (the early name of what is now called PL/I) leaves the character set undefined, but specifically mentions the break character, and gives RATE_OF_PAY as an example identifier. By 1967 the underscore had spread to ASCII, replacing the similarly shaped left-arrow character, ← (see also: PIP). C, developed at Bell Labs in the early 1970s, allowed the underscore in identifiers. Underscore predates the existence of lower-case letters in many systems, so often it had to be used to make multi-word identifiers, since camelCase (see below) was not available.

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