  Under the principle that quantum mechanical observables are invariant under
relevant symmetry transformations, we explore how the usual, non-invariant
quantities may capture measurement statistics. Using a relativisation mapping,
viewed as the incorporation of a quantum reference frame, we show that the
usual quantum description approximates the relative one precisely when the
reference system admits an appropriate localisable quantity and a localised
state. From this follows a new perspective on the nature and reality of quantum
superpositions and optical coherence.
