I needed to set up my trusty Sceptre 27" monitor as a secondary monitor

It works out-of-the-box with ubuntu's stock WM, but who wants to do that?

I found somebody elses's notes on this, but here's mine

 
 xrandr 
 # eDP-2 connected primary 1920x1080+1920+0 (normal left inverted right x axis y axis) 382mm x 215mm 
 ... 
 # HDMI-1-0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 526mm x 296mm 
 # so, the laptop's on-board screen is eDP-2, the external monitor is HDMI-1-0 
 xrandr --output eDP-2 --primary --output HDMI-1-0 --left-of eDP-2 
 

With i3, numbered screens are assigned to different monitors

I only use 3 virtual screens, so this seems like it will work.