  We introduce columnar shifts---collective rigid-body translations---as a
structural degree of freedom relevant to the phase behaviour of molecular
perovskites ABX$_{\textrm3}$ (X = molecular anion). Like the well-known
octahedral tilts of conventional perovskites, shifts also preserve the
octahedral coordination geometry of the B-site cation in molecular perovskites,
and so are predisposed to influencing the low-energy dynamics and displacive
phase transitions of these topical systems. We present a qualitative overview
of the interplay between shift activation and crystal symmetry breaking, and
introduce a generalised terminology to allow characterisation of simple shift
distortions, drawing analogy to the "Glazer notation" for octahedral tilts. We
apply our approach to the interpretation of a representative selection of azide
and formate perovskite structures, and discuss the implications for functional
exploitation of shift degrees of freedom in negative thermal expansion
materials and hybrid ferroelectrics.
