The horizontal surface on which points on a spherical projection can be represented in two dimensions. A stereographic projection can be compared to a slice through the Earth’s equator as seen from the N. or S. Pole, and for this reason is often called the ‘equatorial plane’, and its circumference is called the ‘primitive circle’. Since the plane passes through the centre of the sphere its circumference is a great circle, the angle between the plane of projection and the N–S axis of the sphere being 90°.