A hypotrochoid is a curve traced by a point inside a circle that is rolling round the inside of another fixed circle. This results in interesting patterns of different designs, depending on the radius of the rotating circle and the position of the point inside it.

Apparatus for drawing such patterns is commonly available as a toy or novelty item. It usually consists of gear-like plastic discs with holes in them, which you can place on paper, insert a pencil or pen in and rotate. This program does the same thing on a computer.

You can download HypoTrochoid here.