  We study the geometry of pseudo-Riemannian manifolds which are
Jacobi--Tsankov, i.e. J(x)J(y)=J(y)J(x) for all tangent vectors x and y. We
also study manifolds which are 2-step Jacobi nilpotent, i.e. J(x)J(y)=0 for all
tangent vectors x and y.
