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A directory is an element of the organization of the data in a mass storage system, which records which files are stored in that system, along with some information about them. They exist primarily in file systems, where they typically contain collections of related files, but they are to be found in other mass storage systems as well, such as on magnetic tapes.

Originally, along with the information as to which blocks were part of a file, directories contained other meta-information about the file (date last modified, etc); this approach was used in early operating systems such as CTSS and TOPS-10, as well as later ones like Multics and Windows.

UNIX started a new approach, in which most information about a file (particularly, which blocks were used to hold it) was kept in a separate structure (called an inode in UNIX); directories (usually implemented as an abstraction on top of that file layer) only held mappings from file-names (visible to users) to inode numbers.