Drive letter assignment is the process of matching volumes (active primary partitionss and logical partitions) to letters in the root namespace of a file system as seen by a Microsoft operating system. The concept of drive letters originated in the CP/M operating system, in imitation of the device prefixes in the RSX-11 and VMS operating systems.

Unlike the concept of Unix mount points, where the user can create directories of arbitrary name and content in the root namespace, drive letter assignment implies that only letters are in this namespace, and they represent solely volumes. In other words, it is a process of naming the roots of the "forest" that represents the file-system (with each volume being an independent tree in it).

Operating systems that use drive letter assignment:

Each of these operating systems assign drive letters according to the following algorithm:

  1. Assign the drive letter 'A' to the boot floppy, and 'B' to the secondary floppy
  2. Assign a drive letter, beginning with 'C' to the first active primary partition recognized upon the first physical hard disk drive
  3. Assign subsequent drive letters to the first primary partition upon each successive physical hard disk drive, if present within the system.
  4. Assign subsequent drive letters to every recognized logical partition beginning with the first hard drive and proceeding through successive physical hard disk drives, if present within the system.

In the operating systems below, the assignment nominally follows the algorithm above, but can be manually changed by the system administrator: