A coordinate system defined by three families of surfaces, as follows: (a) longitude is defined by a family of numbered planes intersecting in a common line (the polar axis); (b) latitude is defined by a family of numbered, confocal hyperboloids with common axis on the polar axis; and (c) distance is defined by a family of numbered, confocal spheroids (rotational ellipsoids) having one of their common axes on the polar axis and their common centre on the centre of the family of hyperboloids. If the spheroids are oblate, the latitudes are given by single-sheet hyperboloids with common axis of rotational symmetry on the polar axis. If they are prolate, latitudes are given by two sheet hyperboloids.