  Voronoi defined two polyhedral partitions of the cone of se\mi\de\fi\nite
forms into L-type domains and into perfect domains. Up to equivalence, there is
only one domain that is simultaneously perfect and L-type. Voronoi called this
domain {\em principal}. We show that closure of the principal domain may be
identified with a cone of cut submodular set functions. Parallelotopes of the
closed principal domain are zonotopes that are base polyhedra related to
graphic unimodular sets of vectors.
