setting primary monitor in Ubuntu

vijayp@ike:~$ xrandr --output HDMI1 --primary

In case you don't know what your display is called, just run "xrandr" on its own. This is what my laptop says:

vijayp@ike:~$ xrandr
Screen 0: minimum 320 x 200, current 2880 x 1200, maximum 8192 x 8192
VGA1 disconnected (normal left inverted right x axis y axis)
LVDS1 connected 1280x800+0+0 (normal left inverted right x axis y axis) 261mm x 163mm
   1280x800       60.0*+
   1024x768       60.0
   800x600        60.3     56.2
   640x480        59.9
HDMI1 connected 1600x1200+1280+0 (normal left inverted right x axis y axis) 367mm x 275mm
   1600x1200      60.0*+
   1280x1024      75.0     60.0
   1152x864       75.0
   1024x768       75.1     60.0
   800x600        75.0     60.3
   640x480        75.0     60.0
   720x400        70.1
DP1 disconnected (normal left inverted right x axis y axis)