It is used as a diacritic mark in some African and Native American languages. The underscore is sometimes used as an underdot (Unicode: COMBINING DOT BELOW) in romanized Arabic and Hebrew.

It is also often used instead of a space in computer operating systems and on the Internet in World Wide Web addresses.

Some applications will automatically underline text surrounded by underscores: _underlined_ will render underlined.

It is not a dash, and should not be used as such, or to create a horizontal line.