  An argument that relaxor ferroelectricity in the isovalent alloy $\mathrm
{Ba(Zr}_{1-x}\mathrm{Ti}_{x})\mathrm{O}_3$ can be understood as an induced
moment soft pseudo-spin glass on the B-ions of the $\mathrm{ABO}_{3}$ matrix is
extended to the experimentally paradigmic but theoretically more complex
heterovalent relaxor $\mathrm {Pb(Mg}_{1/3}\mathrm{Nb}_{2/3}\mathrm{)O}_3$
(PMN). It is argued that interesting behaviour of the onset of non-ergodicity,
frequency-dependent susceptibility peaks and precursor nanodomains can be
understood from analagous considerations of the B-ions, with the displacements
of the Pb ions a largely independent, but distracting, side-feature. This
contrasts with conventional conceptualizations.
