Prior to Apollonius, the Greeks derived the conic sections from three types of cones of revolution, according as the vertex angle of the cone was less than, equal to, or greater than a right angle. By cutting each of three such cones with a plane perpendicular to an element of the cone an ellipse, parabola, and hyperbola respectively result. Only one branch of a hyperbola was considered. Apollonius, on the other hand, in Book I of his treatise, obtains all the conic sections in the now familiar way from one right or oblique circular double cone.