The 'Spiro' series: the mathematical relationship behind all these forms is seemingly simple. A circle rotates inside of a larger circle, at 90 degrees to it; that circle rotates inside a still larger circle. A point on the innermost circle traces out the curve. By varying the ratios of the circles and their offsets, a surprising diversity of shapes emerges.
The 'gyro' curve weaves neatly through itself, forming a nearly-spherical shape. Unlike the other figures, it is built from three nested circles, instead of two.