for the Electronic Recording Machine, Accounting (ERMA) Mark 1 published in 1958 as well as by Xerox Star, is used in almost all modern operating systems' desktop environments. Folders are often depicted with icons which visually resemble physical file folders. There is a difference between a directory, which is a file system concept, and the graphical user interface metaphor that is used to represent it (a folder). For example, Microsoft Windows uses the concept of special folders to help present the contents of the computer to the user in a fairly consistent way that frees the user from having to deal with absolute directory paths, which can vary between versions of Windows, and between individual installations. Many operating systems also have the concept of "smart folders" or virtual folders that reflect the results of a file system search or other operation. These folders do not represent a directory in the file hierarchy. Many email clients allow the creation of folders to organize email. These folders have no corresponding representation in the filesystem structure. If one is referring to a container of documents, the term folder is more appropriate. The term directory refers to the way a structured list of document files and folders are stored on the computer. The distinction can be due to the way a directory is accessed; on Unix systems, is usually referred to as a directory when viewed in a command line console, but if accessed through a graphical file manager, users may sometimes call it a folder. Lookup cache Operating systems that support hierarchical filesystems (practically all modern ones) implement a form of caching to RAM of recent path lookups. In the Unix world, this is usually called Directory Name Lookup Cache (DNLC), although it is called dcache on Linux. For local filesystems, DNLC entries normally expire only under pressure from other more recent entries. For network file systems a coherence mechanism is necessary to ensure that entries have not been invalidated by other clients. See also File folder Concepts Directory structure Filename Filesystem Hierarchy Standard Home directory Root directory Virtual folder Working directory Commands cd (command) chroot dir (command) mkdir pushd and popd pwd tree (command) References External links Definition of directory by The Linux Information Project (LINFO) Computer file systems File system management