RandR (xrandr)
The X11 extension that made it possible to change resolution, rotate and plug in monitors without restarting X.
Resize and Rotate was designed by Jim Gettys and Keith Packard around 2001, when handhelds and laptops made it obvious that a screen does not stay one size forever. Before RandR, a new resolution often meant editing a config file and restarting the server — taking every open program down with it.
Version 1.2 in 2007 was the big one: each output, such as HDMI-1 or eDP-1, became its own object that could be enabled, positioned and rotated on a live system. That let it replace Xinerama. The command-line tool xrandr --output HDMI-1 --auto --right-of eDP-1 became the incantation typed whenever the meeting-room projector refused to cooperate. Under Wayland the compositor handles all this itself.