An underline, also called an underscore, 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. 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.

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