  The large number of low-lying states of d- and f-shells supports a variety of
order parameters. The effective dimensionality of the local Hilbert space
depends on the strength, and kind, of intersite interactions. This gives rise
to complicated phase diagrams, and an enhanced role of frustration and
fluctuation effects. The general principles are illustrated on the example of
the effect of a magnetic field on quadrupolar phase transitions in some
Pr-based skutterudite compounds.
