Dual pendulum drawing machine — Lissajous curves with damping
A harmonograph uses two pendulums — one controlling X, one Y — to draw curves on paper. When the frequency ratio is rational (p/q), the curve is a closed Lissajous figure. Slight detuning (e.g. 2.001 vs 2.000) causes the pattern to slowly precess and evolve before damping stops the motion.