[Linux 常見問題] How to set the primary display on CentOS/RHEL 7 with dual monitors and GNOME

Source From Here 
Preface 
This article will describe a way on how to change/set a primary display on CentOS/RHEL 7 with dual monitors and GNOME. The easiest and perhaps recommended way is to use GUI and navigate to

Applications->System Tools->Settings->Displays. 

How-To 
The current primary screen is highlighted by a black top bar. To change you primary display simply drag-n-drop the black top bar from one screen to another: 
 

Another alternative is to you command line tool xrandr. First get all your connected screens: 

$ xrandr | grep -w connected 
DVI-D-1 connected 1680x1050+0+0 (normal left inverted right x axis y axis) 474mm x 296mm 
HDMI-1 connected primary 1920x1080+1680+0 ([color=brown]normal left inverted right x axis y axis) 598mm x 336mm[/color]

From the output above we can see that there are two connected screens DVI-D-1 and HDMI-1 where HDMI-1 is connected as a primary screen. To set for example DVI-D-1 as a primary display execute the following linux command: 

$ xrandr --output DVI-D-1 --primary

The above command will switch you primary displays. However, this setting will not persists after reboot unless you add you command to you ~/.bashrs file. 

Supplement 
* FAQ - How to see/change screen refresh rate or monitor frequency? 

// First we check available modes. 
$ xrandr 
1440x900 59.9+* 75.0 
1280x1024 75.0 60.0 

// Then we pick the mode, including resolution and refresh rate. 
$ xrandr -s 1440x900 -r 75 

// Or just the refresh rate 
$ xrandr -r 75 

// Let's see if it worked 
$ xrandr 
1440x900 59.9+ 75.0* 
1280x1024 75.0 60.0