Display

• xrandr is X11-only. On Wayland it won't work — check your session first: echo $XDG_SESSION_TYPE (x11 vs wayland).
• Output = a physical connector: eDP-1 (laptop panel), HDMI-1, DP-1. Mode = resolution + refresh rate.
• Software vs hardware brightness: xrandr --brightness dims the signal (fake); real backlight control is brightnessctl / /sys/class/backlight.

xrandr — outputs & modes

Command	What it does
xrandr	list outputs + available modes
xrandr --query \| grep " connected"	just the connected outputs
xrandr --output HDMI-1 --mode 1920x1080	set resolution
xrandr --output HDMI-1 --mode 1920x1080 --rate 60	set resolution + refresh
xrandr --output HDMI-1 --primary	make it the primary display
xrandr --output HDMI-1 --auto	enable at preferred mode
xrandr --output HDMI-1 --off	disable an output
xrandr --output HDMI-1 --right-of eDP-1	position (also --left-of, --above)
xrandr --output HDMI-1 --rotate left	rotate (normal/left/right/inverted)
xrandr --output HDMI-1 --scale 1.5x1.5	scale (software)

Add a custom resolution

Step	Command
generate a modeline	cvt 1920 1080 60
register it	xrandr --newmode "<modeline>"
attach to an output	xrandr --addmode HDMI-1 <name>

Wayland equivalents

Environment	Command
wlroots (Sway, etc.)	wlr-randr
Hyprland	hyprctl monitors
GNOME / KDE	Settings → Displays (GUI)

Brightness (real backlight)

Command	What it does
brightnessctl set 50%	set backlight
brightnessctl g / brightnessctl m	current / max value
cat /sys/class/backlight/*/brightness	raw value

Troubleshooting

Symptom	Check
Monitor not detected	xrandr to see if listed; reseat cable; --auto
xrandr says "can't open display"	you're on Wayland (or no $DISPLAY set)
Wrong refresh rate	xrandr --output X --rate 60

See [[window-managing]] for compositors, tearing, and WMs.

For a real-world case (XFCE's display GUI doing nothing under a bare Openbox session, autorandr, and a toggle script), see [[hdmi-tv-openbox]].