A file is typically a large list i.e. stored in the external memory of a computer (i.e., magnetic disk). In addition, a file may be used as a repository for the items (commonly called records) that is accessed infrequently and/or must be stored between various invocations of a program. In external memory, as opposed to main memory, the time to access different list items may not be equal. As a result, new types of information organizations and access techniques are necessary for files. These new types of organizations, called file structures.