Pendulum
Tilt your phone and it draws — and sings. Two pendulums, one per axis; leaning sets their frequency ratios. When the ratios settle near a simple whole-number relationship the traced figure closes into a still rosette and the two tones lock into a consonant chord. Between those points the figure precesses and the tones beat against each other. Sight and sound agree in one gesture.
manual control
No phone to tilt? Drag on the figure, or set each pendulum here.
brightness
Each strike rings down over ~26 s. Tap Re-strike to swing the pendulums again.
Lock targets are the just ratios 1:1 · 5:4 · 4:3 · 3:2 · 5:3 · 2:1 — unison, major third, fourth, fifth, major sixth, octave. Nothing is quantised: sit between two and you hear the roughness the figure's precession is drawing.