Why can't you create a folder named CON in Windows or MS-DOS?

In MS-DOS, several special "device files" were available to aid in performing certain tasks, such as clearing the screen or deleting extraneous output from a program. In order to maintain backwards-compatibility, all versions of Windows up to and including Windows 7 will refuse to allow you to create a file with these "reserved" device file names. The following file names are reserved:

  • CON
  • PRN
  • AUX
  • NUL
  • COM1
  • COM2
  • COM3
  • COM4
  • COM5
  • COM6
  • COM7
  • COM8
  • COM9
  • LPT1
  • LPT2
  • LPT3
  • LPT4
  • LPT5
  • LPT6
  • LPT7
  • LPT8
  • LPT9

CON, incidentally, was a device file used to capture whatever was printed onscreen.