The underscore key is most commonly used for indicating a space between words in a context where a space is not a permitted character. Examples include website URLs, email addresses, file or folder names, and computer programming.
The underscore was originally created as a key for typewriters to underline words. Typewriters lack the ability to automatically underline words, instead requiring typists to type the word, then backspace and underscore each letter. The underscore is sometimes referred to as underline, low dash, or understrike. Underscores may also be used for fill-in-the-blank type sentences, such as those found on tests, or where space for a signature is needed on a form.