||This article needs additional citations for verification. (September 2012)|
SUBST is a command on the DOS, IBM OS/2 and Microsoft Windows operating systems used for substituting paths on physical and logical drives as virtual drives. It is similar to floating drives, a more general concept in operating systems of Digital Research origin, including CP/M-86 4.x, Personal CP/M-86 2.x, Concurrent DOS, Multiuser DOS, System Manager 7, REAL/32, as well as DOS Plus and DR DOS (up to 6.0).
In Windows NT,
DefineDosDevice() to create the disk mappings.
JOIN command is the "opposite" of
JOIN will take a drive letter and make it appear as a directory.
This is the Command Prompt output under Windows XP:
C:\>SUBST /? Associates a path with a drive letter. SUBST [drive1: [drive2:]path] SUBST drive1: /D drive1: Specifies a virtual drive to which you want to assign a path. [drive2:]path Specifies a physical drive and path you want to assign to a virtual drive. /D Deletes a substituted (virtual) drive. Type SUBST with no parameters to display a list of current virtual drives.
This means that, for example, to map C:'s root to X: one would use
SUBST X: C:\ at the command line. Upon doing this, a new drive called X: would appear under the My Computer virtual folder in Windows Explorer.
To unmap the drive, type in the following command at the command prompt:
C:\>SUBST [drive:] /D
Custom label 
A custom label can be assigned to a drive letter created in this way by way of a registry key, which can be created by renaming (select "rename" from the drive letter context menu or press F2) the SUBST drive in Windows Explorer/My Computer.
(DefaultValue) = Your Drive Label
"M" represents the drive letter to assign a custom label to.
However, labels created for SUBST drives in this manner are overridden by the label of the host drive/partition: the custom labels are only used if the host drive has no label. One may then:
- Delete the host's drive label;
- Create the proper registry keys for the SUBST drive letter;
- Create the proper registry keys for the host drive letter (optional, works around the host drive label override caveat);
- Re-create the SUBST drive to see label changes applied.
Persisting across reboots 
Drive letters mapped in this way are not available during system startup for services nor do they persist across a reboot. However with a registry modification it is possible to assign a path to a drive letter during startup so it is available to system services and persists across a reboot.
Create a new registry entry "String Value" in the following key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\DOS Devices
The name should be "X:" where X is the drive letter.
The value should be the path in the form:
or in form:
or in form:
There are tools available to make the necessary changes for you.
The relative to this thematic registry key is
HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices It defines mapping of drive letters into particular hard disk partitions, similar to /etc/fstab on a Unix system. It also can be edited manually, but only at that time while that particular installed Window operation system is "inactive". So that, for example, if you currently boot from "
D:\Windows" then you can edit the
HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices key of Windows that is installed in a "
C:\Windows" folder, for an instance by doing the following actions:
- run command:
reg load hklm\$system C:\WINDOWS\system32\config\system
- run command:
- edit registry key
(that represents the
HKEY_LOCAL_MACHINE\SYSTEM\MountedDevicesregistry key of the Windows instance that is installed in
- run command:
reg unload hklm\$system(to complete the editing procedure)
- Windows actions which act on disks at the physical layer are not possible.
- Since (at least) Microsoft Windows XP SP2 AUTORUN.INF files present in the new drive letter are ignored; thus, AutoRun/AutoPlay does not work on drive letters created in this way. In addition, assigning a custom icon or label to the drive letter created this way via AUTORUN.INF does not work. A custom label assigned to the drive letter created with
SUBSTonly appears if the source drive/volume does not have a volume label set (check and set with the
See also 
- fsutil hardlink create - creates a hard link (Windows 2000 and later)
- Microsoft.com - to create a symbolic link or hard link (Windows Vista and later)
- SS64.com about "subst"
- psubst tool for persistent mappings
- Visual Subst — a GUI tool with persistent drive-mapping option.