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. 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). The combining diacritic, ◌̱ ( macron below), is similar to the combining low line but is shorter.

It is also used as markup to indicate underscore or italics, for instance _thus_ rendering as thus or thus. The underscore character is used in strings where spaces are not permitted to create visual spacing, such as in computer filenames, email addresses, and in Internet URLs, for example Mr_John_Smith. In modern usage, underscoring is achieved by markup or with the Unicode combining low line. 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. The underscore character, _, also called a low line, or low dash, originally appeared on the typewriter so that underscores could be typed. Its use to add emphasis in modern finished documents is generally avoided.

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

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