I cannot locate it in a clean install of Linux Mint 18.3 cimammon.

probably not needed at all

Modern versions of xorg usually configure themselves automatically, with sensible defaults. You might need a keymap file in /etc/X11/xorg.conf.d if you're not using an American keyboard.

The keyboard is fine.
However I have two screens. To get to the left i have to move the mouse to beyond the extreme right.

I do not have nvidia cards, just
AMD/AT1 cards.

Introduction of an xorg.conf from a Mageia system caused Cinnamon to crash.
Editing the file (badly it appears) caused Mint not to come up at all!
I am going to have to boot on a live CD to edit it (by hit and miss) again. painful.

You're going about this in the totally wrong way. As stated by others, xorg.conf is not needed any longer, and only if you have a VERY exotic configuration (as hazel intimated, about keyboards, or with exotic mice/pointing devices/screens), is any manual tweaking necessary. Even then, those things are handled through the appropriate files in /etc/X11/xorg.conf.d directory. You have two easy choices:

• In whatever window manager you're using (you say Mint, Cinnamon), you can go to "Menu->Preferences->Displays", and you should see both monitors. Set them the way you like. Sounds like they're both detected, so you should just be able to drag/drop, and save your changes when done.
• Swap the cables between the monitors.

I object! There's nothing exotic about using a British (or other European) keyboard layout. American keyboards have everything in the wrong place!

Hazel, I was not saying a British keyboard was exotic; I meant exotic HARDWARE; like many multimedia keys/old vs. very new hardware, gaming keyboards and the like. Things like mice with 30 buttons for gaming. Keyboards with specialized keycard readers or fingerprint sensors. Understand? And while you may think an American keyboard is wrong, others likely think the same of a British one.

Sadly "Menu->Preferences->Displays" is not as flexible as you appear to suggest.

I want my primary screen to remain on the right and my secondary on the left.

I want my desktop icons and customised panel to remain on the right.

This last section of my xorg.conf (below) does what I want in Mageia et al but it cannot stand alone. The labels in it must be defined. They are of course in the full file.

Section "ServerLayout"
Identifier "layout1"
Screen "screen1"
Screen "screen2" LeftOf "screen1"
#Option "Xinerama"
EndSection

Oh? How so? Because in KDE and Gnome, it *IS* as easy as moving displays with the mouse.
Ok, so you should do that, then.
Again: xorg.conf **IS NOT** used any longer. While it WILL work, having one present causes all sorts of problems, if it's not configured correctly. Again, the configuration files you need are in /etc/X11/xorg.conf.d If you already have put an xorg.conf file in place, remove it and restart your system, THEN try to move the screens around. What you're doing would have been great ten years ago (or longer), but not any longer. The use of xorg.conf is for legacy use only.

I have not had to use one in at least seven years, and even then it was on an older system. As long as your system recognizes your video hardware, and can detect two screens, you don't need xorg.conf in any way. Again, if you need to define exotic hardware (see previous comments), you do that in /etc/X11/xorg.conf.d

i want, i want...

please read and formulate a command that does evrything you want.

after that we can help you make it so that this command is automatically executed at login.

and i concur, you should not need a xorg.conf to satisfy your wants.

1 members found this post helpful.

And I'll also add to this by again pointing out that, since you find this so difficult, you could just SWAP THE CABLES on the monitors, and be done, right?

Putting that xorg.conf file on your machine is not a good idea, especially since you copied it from another machine with different hardware. Again, that's why you can't drag-n-drop your monitors, since xorg.conf takes precedence. We're trying to help you, but if you don't want to follow the advice given, there's little we can help WITH.

Although it is most probably completely offtopic, but no!
I use an ubuntu 14.04 (with gnome) with two identical HP monitors. When it booted I had to exchange the monitors (setting->displays) because "by default" the left monitor was "on the right side" of the other.
I did swap the cables (actually two identical HDMI cables) and now the system somehow recognized this and the left monitor is still (or again) on the right side.
Probably it remembers the configuration and restores it during boot, just it is a bit buggy....

I've not seen that behavior, but it honestly wouldn't surprise me. Years ago when you HAD to have an xorg.conf file, you'd have to spend a good long time getting multiple displays working.

I'm using an iMac as my main desktop machine now. 27" retina display, and compact on my desk. Plugged in a second monitor into the displayport plug, and it 'just worked', zero configuration. Went into KDE settings, told it how I wanted things, and that was that. Much easier, and with hardware that's fairly picky. A 'standard' card should work fine, but the OP did mention that it was an ATI card. And ATI support isn't that great. OP hasn't said what kind of card, but some older ones (some HD series, etc.), have ports that don't work with the radeon driver, because ATI doesn't like to play nice with Linux. The newer amdgpu drivers work pretty well, unless you've got a REALLY new ATI card.

I thought I was clear. Mint has corrdtly identified which monitor is which and selected the cottect one as the primary.

The only problem is that to move the mouse from the right screen to the left requires that it has to be moved to the right(!)

I'll run up xrandra when I next gain access to the system.

Thank you for your advice.

Sorry, but are you intentionally ignoring what's being said?

We KNOW you have both monitors working...which is why moving the cables is a quick 'fix'. We also know that an xorg.conf file is a **BAD IDEA**, and we have told you so, and we've told you to delete it, THEN try to make Cinnamon move your screens/set defaults. The xorg.conf WILL CONFLICT with things.

Again, there is no need for it. One from another system won't work, and wouldn't even when that file WAS used. Running xrandr will let you set aspects of your X configuration, but AGAIN, there is not a need to do it. xrandr is like xorg.conf...a remnant. If your screens are WORKING, there's no need to set hsync/vsync/aspect/etc, via xrandr.