Any pair of (possibly unalike) objects with well-defined centers can be concentric, including circles, spheres, regular polygons, regular polyhedra, parallelograms, cones, conic sections, and quadrics.
Geometric objects with a well-defined axis include circles (any line through the center), spheres, cylinders,[2] conic sections, and surfaces of revolution.
[11] Johannes Kepler's Mysterium Cosmographicum envisioned a cosmological system formed by concentric regular polyhedra and spheres.
[12] Concentric circles have been used on firearms surfaces as means of holding lubrication or reducing friction on components, similar to jewelling.
Spheres: Apostol (2013)Regular polygons: Hardy, Godfrey Harold (1908), A Course of Pure Mathematics, The University Press, p. 107Regular polyhedra: Gillard, Robert D. (1987), Comprehensive Coordination Chemistry: Theory & background, Pergamon Press, pp.