i3 is my favourite window manager (and desktop environment). When I connect an external monitor or projector to my laptop, though, xrandr comes to the rescue on how they should work with each other
By default when an external monitor, say VGA-1, is connected to the laptop, i3 puts one workspace on that. If it does not do it automatically, the way to do it is with:
$ xrandr --output VGA-1 --right-of LVDS-1 --auto
This puts VGA-1 to the right of LVDS-1 (which maybe the name of the internal monitor of the laptop).
What if I just want to fold my laptop and use i3 completely on the external monitor or projector?
$ xrandr --output LVDS-1 --off --output VGA-1 --auto
How to turn the internal monitor back on?
$ xrandr --output LVDS-1 --auto
Now, what if I still want one workspace on the internal monitor?
$ xrandr --output LVDS-1 --right-of VGA-1 --auto