the basic index concept

Computer Science An Overview _J. Glenn Brookshear _11th Edition

Over the years numerous variations of the basic index concept have been used.
One variation constructs an index in a hierarchical manner so that the index takes on
a layered or tree structure. A prominent example is the hierarchical directory
system used by most operating systems for organizing file storage. In such a case, the
directories, or folders, play the role of indexes, each containing links to its subin-
dexes. From this perspective, the entire file system is merely one large indexed file.