underscores

The underscore is a typographic symbol represented by the character _ (ASCII code 95, Unicode name "LOW LINE"). It is used as a spacing or joining mark in various digital and writing contexts, where a visible separator is needed in place of a space.

In typography and writing, underscores often serve as a stand-in for spaces in environments where spaces are not allowed or visible, such as in file names, usernames, or identifiers. They can also appear in plain text to imply emphasis or to imitate underlining in systems that do not support rich formatting.

In computing, underscores are widely permitted in programming identifiers. They are used to join words in names (for example, snake_case) and to convey meaning in naming schemes such as constants written in all caps (SCREAMING_SNAKE_CASE) or internal variables. Underscores are also practical in languages that restrict spaces in identifiers. In many programming languages, a leading or trailing underscore can have a conventional or syntactic purpose, such as signaling a private or unused variable.

In mathematics and typesetting, the underscore has a distinct role: in TeX and LaTeX, it denotes subscript in math mode. It also exists as a regular character in certain contexts, separate from its spacing or subscript function.

Variations and related concepts include the hyphen and the space as separators in identifiers and file names, naming conventions that influence readability, and the broader use of underscores in code style guides and documentation.