When you have a dual head display one of them is the “primary monitor”. Usually, window manager put special panels or content in that monitor (gnome-shell).
To see which are your connected monitors (and their names) run:
$ xrandr | grep connected
DVI-I-0 disconnected (normal left inverted right x axis y axis)
VGA-0 disconnected (normal left inverted right x axis y axis)
DVI-I-1 connected primary 1920x1200+0+0 (normal left inverted right x axis y axis) 530mm x 300mm
HDMI-0 connected 1920x1200+1920+0 (normal left inverted right x axis y axis) 530mm x 300mm
In this case my primary monitor is DVI-I-1. To change to the other available monitor, run:
$ xrandr --output HDMI-0 --primary
Cheers
