Windows 10 Still Won’t Let You Use These File Names Reserved in 1974
 

Windows “reserves” certain file names and doesn’t let you use them anywhere. Forget calling a file “con.txt” or “aux.mp3”. This is all because of a choice made in 1974 and Microsoft’s thirst for eternal backwards compatibility.
File Names You Can’t Use

Microsoft provides an official list of reserved file names, and here they are:

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

Note that you can’t use these file names with any file extension. So you can’t name a file “con.txt”, “con.jpg”, “or “con.doc”. And Windows isn’t case sensitive, so it doesn’t matter whether it’s CON, con, or CoN—Windows won’t let you use that name.

You can try it yourself. Try saving a file as “con.txt” or “lpt6.txt” in Notepad. Or try renaming any file to one of these names in File Explorer. Windows just won’t let you do it.

Of course, Windows also restricts file names in other ways. You can’t use various special characters like the following in names, either. Consult Microsoft’s official documentation for more information.
https://www.howtogeek.com/fyi/window...erved-in-1974/