  A theory that fully describes previously observed neutron Bragg spots
indicative of magnetic order in the pseudo-gap phase of YBa2Cu3O6 + x is
presented. Magnetic diffraction intensities, unveiled by polarization analysis,
are explained by parity-violating magnetic (magneto-electric) quadrupoles at
in-plane Cu sites arranged in a simple ferro-type motif. An absence of
conventional spin and orbital angular momentum dipoles in the theory is
compatible with the occurrence of superconductivity and strongly suggests,
moreover, that magneto-electric quadrupoles are the primary order-parameter in
the pseudo-gap phase.
