  We completely describe the signatures of the Ricci curvature of
left-invariant Riemannian metrics on arbitrary real nilpotent Lie groups. The
main idea in the proof is to exploit a link between the kernel of the Ricci
endomorphism and closed orbits in a certain representation of the general
linear group, which we prove using the `real GIT' framework for the Ricci
curvature of nilmanifolds.
