Under DOS, OS/2, and Microsoft Windows, each partition has a drive letter assignment (labeled codice_4 for a particular partition C) and there is no common root directory above that. DOS, OS/2, and Windows do support more abstract hierarchies, with partitions mountable within a directory of another drive, though this is rarely seen. This has been possible in DOS through the command codice_5 since it first was added to DOS, and can be achieved in all Windows versions as well. In some contexts, it is also possible to refer to a root directory containing all mounted drives, although it cannot contain files directly as it does not exist on any file system. For instance, when linking to a local file using the "file:" URI scheme, the syntax is of the form "codice_6", where "codice_7" is the standard prefix, and the third 'codice_1' represents the root of the local system.