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.
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