"Just as the geometric properties of a circle are determined by its radius (a single distance), so are all the geometric properties of an ellipse determined by two distances: its semimajor axis a and its semiminor axis b. We say that the circle has a single geometrical degree of freedom, whereas the ellipse has two such degrees of freedom. ...The larger b is for a given value of a, the rounder the ellipse, and the smaller b is for a given value of a, the flatter the ellipse. If b = a, the ellipse is a circle and if b = 0, the ellipse is a straight line. ...its geometrical properties are a kind of average of the geometrical properties of a circle with radius a and a circle of radius b. ...The arc of the ellipse lies between the circumferences of these two circles; it is inscribed in the larger one and circumscribes the smaller one. ...the ellipse is symmetrical with respect to the y and x axes."