Double pendula side-by-side for demonstration of
- sensitivity to initial conditions (SIC),
- rate of divergence depends on the total energy of the system (Lyapunov exponent),
- existence and shadowing of periodic orbits (POs),
i.e. some fundamental properties of chaotic dynamics.

Slider sets and displays the magnitude of difference between the pendula,
which increases as time progresses.
When viewing POs, the slider sets the maximum length of nearby POs displayed.

Drag pendulum to starting position, then press play.
If displaying POs, this will select a PO as an initial condition.

Slow-motion option in settings.

A clean and simple demonstration piece.
(By the author of openpipeflow.org)