A note that never stops. Play scales and long tones against it and your ears learn where "in tune" actually lives — the practice tool string teachers assign and nobody's phone has.
Pick the key of your piece, start the drone quietly, and play slow scales over it. When a note is truly in tune with the drone, the combined sound goes still — when it's off, you hear beating, a slow wobble. Chase the stillness. Five minutes a day of this builds intonation faster than any gadget, including our own tuner — which is the right tool for strings and pegs, while the drone is the right tool for your hands and ears.
The "rich" character adds a fifth above the root, which makes thirds and sevenths easier to place. Singers: start with pure.