Confocal (blue), constant-depth (red), and co-axial (black) coated or mantled spheroidal cores (green) shown in cross-section. The spheroid semi-axes are a, b, and c′, where the c′ axis is perpendicular to the plane of the figure. For the confocal case, the linear eccentricity $c=\sqrt{}(a^2-b^2)$. The coloured symbols indicate the foci of the elliptical cross-sections for the core and core+mantle.