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Ubuntu 15.10.

I have three monitors connected to this computer, that works fine. Except......

How do I know which monitor Ubuntu considers to be the primary desktop? And, how do I control that?

Thanks.

UPDATE: Well, after all the help of the nice folks below, and more searching around, I found a PARTIAL answer to my original question: arandr. If you run arandr, it very clearly allows you to specify which display is the primary. However - this is why this is only a partial answer in my case - no matter which I selected via arandr to be the primary, the acting primary monitor stubbornly remains the leftmost monitor.

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  • xrandr --primary - see blog.bodhizazen.net/linux/use-xrandr-to-set-a-screen-resolution . We need the output of xrandr -q to be more specific
    – Panther
    Mar 4, 2016 at 22:15
  • As mentioned above, no need to install arandr, xrandr is perfectly capable of doing the job to set or get the primary monitor. It is not clear however what is exactly the concept of "primary monitor" you have in mind.
    – Jacob Vlijm
    Mar 7, 2016 at 16:45

2 Answers 2

Reset to default
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Settings --> Displays

enter image description here

If you want more advanced options

sudo apt-get install unity-tweak-tool

Lets call it an Add-on which gives you more options than setup :).

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  • yes, I already had that installed. But, that really doesn't address my question: How does the system - specifically in your suggestion, how does unity-tweak-tool "decide" which of the three monitors is the primary monitor?
    – dln949
    Mar 6, 2016 at 20:31
  • Take a look at this: superuser.com/questions/546069/…
    – Izzno
    Mar 6, 2016 at 20:44
  • Thank you. However, my system does not have an xorg.conf file for me to modify. And, as far as modifying the ~/.config/monitors.xml file..... I could find nothing in that file that enabled me to identify which monitor is which.
    – dln949
    Mar 7, 2016 at 1:47
  • The only suggestion i have left is to make sure the monitor you align as Center is connected to Port1 (ex: Hdmi 1) Left:hdmi2 Center:hdmi 1 Right:hdmi 3.
    – Izzno
    Mar 7, 2016 at 7:47
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Sorry if I was unclear. What i meant was, what exactly do you define as the primary monitor? I am aware of those settings, but they only give you the options of putting the launcher on certain monitors and not others, and it lets you change what resolutions you want to use for the monitors. As such, there is no primary monitor, there are options which may make it look primary, but really, the difference is in what each monitor looks like.

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