Those are reserved file names from the DOS days. There are a few more.
Below is a list of default device names.
CON Keyboard and display
PRN System list device, usually a parallel port
AUX Auxiliary device, usually a serial port
CLOCK$ System real-time clock
NUL Bit-bucket device
A:-Z: Drive letters
COM1 First serial communications port
LPT1 First parallel printer port
LPT2 Second parallel printer port
LPT3 Third parallel printer port
COM2 Second serial communications port
COM3 Third serial communications port
COM4 Fourth serial communications port
For more information, read this article:
What's the deal with those reserved filenames like NUL and CON?