The Magnetic Structure of $E{r}_{2}T{i}_2{O}_7$

We employ the previously published neutron scattering experiments\cite{1,2,3} to suggest that Er${}_2$Ti${}_2$O${}_7$ has a broken-symmetry multiple-{\bf q} state with tetragonal magnetic symmetry\cite{4}. The ordered moments do not appear to lie close to the crystal-field anticipated directions\cite{5} and we suggest that the low energy gapless mode, visible in specific-heat measurements\cite{6}, is not of the usual transverse isotropic Goldstone-mode type, but is longitudinal and is associated with the internal transfer of magnetism between distinct magnetic Bragg spots.