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.