Query the displays for their names and available modes,
xrandr -q
To reset the displays,
xrandr --auto
Shows each display's optimal (+) and current (*) resolution "mode". LVDS is the laptop screen, and eg. CRT1 is the external display.
To (turn on) and change the resolution,
xrandr --output SCREEN_NAME --mode WIDTHxHEIGHT
To position one display relative to another,
xrandr --output SCREEN1_NAME --right-of SCREEN2_NAME
- --above
- --below
- --right-of
- --left-of
(Some setups may not be possible.)
To sort a screen first in RandR and Xinerama,
xrandr --output SCREEN1_NAME --primary
xrandr --output LVDS --mode 1280x720 --pos 0x-720 --output VGA1 --mode 1280x720 --pos 0x0