This model illustrates the difference between the oscillations of the circular pendulum and the cycloidal pendulum.
The pendulum is a physical system composed of a cord and a weight, whose oscillations take place in equal intervals of time (isochronism).
The oscillations of a circular pendulum (which describes arcs of a circumference) are however isochronous only when they are rather short. The oscillations of a cycloidal pendulum (which describes arcs of a cycloid) are isochronous regardless of their amplitude.