Updated: Jun 5, 2020
I've seen a few posts about this subject of late so I thought I'd join in.
It probably doesn't occur to most people these days, but back in the early days of computing certain words, or commands, were reserved for special functions.
Because nothing was graphical - no mouse, no pretty picture or icons, everything was done with text.
So when the system had to do something special some words were reserved.
This meant that we, the users, couldn't use them.
The one most of us probably hit up against would be LPT1 - this was reserved for printers (Line PrinTers specifically).
So, if for some bizarre reason you wanted to name your document LPT1.doc you were out of luck.
You may indeed be wondering why, with a world of possibilities, would someone limit the number of characters they had in the title of a document? Well, it wasn't a choice. It just wasn't technically possible.
Back in those days documents and filenames could ONLY be 8 characters long with a 3 character extension.
The problem with short names is that they are more easily ambiguous.
I had a customer once who hired a temp' to do the office work.
The temp had worked long and hard for a number of months and had diligently put all of their work into a folder called "temp"...obviously.
...and, some of you are ahead of me already, along came the IT guy and deleted the temp (read: temporary) folder because the machine was low on space.
NOTE: It was the IT person's habit of choice to create a "temp" folder to put things in that were temporary and readily available for deleting if and when another IT person comes along and needs the space. I still do this today. It is where I put stuff I use once and can find again from somewhere else if needed. So still DON'T name anything important TEMP.
The full list of this archaic but still reserved words is as follows:
CON, PRN, AUX, NUL, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9
These things still exist today because of the need to be backwardly compatible with certain old software and conventions.
Combine that with the fact that certain symbols on the keyboard also had/have special meaning in DOS and you can imagine how easy it is for things to go wrong and how hard it is for old guys like me to suddenly have to include those same symbols in passwords.
A list of those and there use can be found here: https://gerardnico.com/lang/dos/character
Every now and then I still bump up against a weird filename issue because someone has used a weird symbol, or copied a file from one operating system to another, without regard for any underlying effects. I mean, why would they? Who knew?
"DOS" is still kinda alive and well. It is within the COMMAND console and Power Shell available in all modern computers - PCs, Macs, etc.
An article explaining the reserved filenames in alternate detail can be found here:
A recent instance of what I am describing in this post can be found here:
An entire database erased because of a few characters accidentally in the right/wrong combination. Fantastic!! 😂