Windows "reserves" certain file names and does not allow you to use them. You cannot name a file as "con.txt" or "aux.mp3". All of this is due to a choice Microsoft had made in 1974 and to the company's persistence for compatibility with past software.
This is the list of names that Microsoft does not allow you to use:
CONN, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8 and LPT9
Note that you cannot use these file names with any extension file. Therefore, you cannot name a file “con.txt”, “con.jpg”, “or” con.doc”. And Windows is case insensitive, so it doesn't matter if it's CON, con or CoN. Windows will not let you use this 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 restrict the naming of files in other ways. Like, you can not use different special characters. Consult it Microsoft official article for more information.
What happened to 1974?
The theme with the naming restriction dates back, as we said, to 1974. Back then, UNIX where “everything was a file” prevailed (The same goes for UNIX-like operating systems, such as Linux today.) The devices of then computers they had file names. If you wrote a text and wanted it to be printed, you simply saved it as a corresponding printer file.
In 1974, this same concept was added to the CP/M operating system, which was aimed at extremely small computers without hard drives. In turn, this idea was passed on to PC-DOS. In 1983 PC-DOS 2.0 added the directories, but Microsoft chose to have these device files appear in all directories, for compatibility with the existing software DOS, rather than placing them in a special devices folder.
Eventually, later Windows 95s were built on DOS. Windows NT was not DOS-based, but they wanted to be compatible with Windows 95 applications. Windows 10 still based on Windows NT works the same way. The same goes for Windows 7, of course.
Now, over forty years later, we still can't name our files as "con.txt" or "aux.mp3" because Windows wants to remain compatible with ancient programs that may use this feature. It's a good example of how strongly Microsoft has been committed to compatibility with the past.
