The mechanical system of two disks, moving freely in the plane,
while in contact and rolling against each other without slipping,
may be written as a Lagrangian system with three degrees of freedom
and one holonomic rolling constraint. We derive simple geometric
criteria for the rotational relative equilibria and their stability.
Extending to three dimensions, we derive the kinematics of the
analogous system where two spheres replace two disks, and we verify
that the rolling disk system occurs as a holonomic subsystem of the
rolling sphere system.