Spiro draws a curve similar to the patterns that could be made with the Spirograph toy that was popular back in the 1960s. Although the curves are similar, the way in which the curves are specified is different.
In Spiro, you specify the parameters for a set of wheels: radius, teeth, and offset. The axis of the first wheel is at the center of the drawing area; the axis of the second wheel is on the perimeter of the first wheel, and moves around that perimeter as the first wheel rotates. The axis of the third wheel (if it exists) is on the perimeter of the second wheel, and moves around the perimeter of that wheel as it rotates as well as moves. The drawing pen is attached to the perimeter of the last active wheel, and moves around the perimeter of that wheel as it rotates and moves. The sum of all of these rotations is what produces the curves.
It sounds complicated, but is really pretty simple. The best way to understand what is happening is to turn on the animate option in the control panel, then press the Draw button.
The curve produced by this motion is variously called a roulette, trochoid, or just plain spirograph by us laypeople.
Back to Spiro by Jimmc