The underscore is a special symbol that is used in computer programming languages, in file names, and in URLs on the Internet. It is mostly used instead of the space key because many formats do not accept an empty space within the variable or name. The underscore was developed as an acceptable virtual space that could be used instead. This symbol is also known as a low dash, understrike, or low line.
When programming languages evolved, there was a dilemma as to how to represent strings of characters in an understandable way without using spaces. For instance, "thisisanobject" as the name of an entity or identifier within a programming language would difficult to read and understand, especially when used repeatedly at different points. Some programming languages allowed a hyphen to be used to separate the word, such as "this-is-an-object." Unfortunately, quite a few languages treated the hyphen as a subtraction operator, and their syntax wouldn't allow it to be used as a separator.
Empty spaces couldn't be used within the identifier, and there was a real need for a symbol or character that would literally fill the gap. In the late 1960s, the underscore was included as an alphabetic character, making it easier for programmers to name objects with multiple words. This symbol is used to concatenate very large string names and to create readable variable names. Using the previous example, "this_is_an_object" would be a valid variable name accepted by many programming languages.
E-mail names, URLs on the World Wide Web, and very long file names within the computer's operating system use the underscore to separate words. File names, songs, and Word documents can be saved as "name_name_name," for instance. It can also be used to separate the words in user names, for example, when logging onto the computer. It is found on a keyboard near the top next to the zero on the same key as the hyphen. Holding down the shift key while pressing the hyphen key results in an underscore.
The underscore has a very interesting history. It was used first in typewriters to emphasize certain words by underlining them. At that time, it was the only way to express printed words with any sort of inflection. The underscore emerged as a simple and inexpensive way to underline printed words on the typewriter. It is sometimes used to create blank spaces in forms by creating a long horizontal line.