Abstract When a mineral is included in another, the pressure on the two is the same. As pressure and temperature then change, the pressure on the inclusion is likely to become increasingly different to the pressure on its host as a consequence of the two minerals having different equations of state (i.e. pressure–volume–temperature P – V – T relationships). Under some limiting assumptions, including that the host behaves elastically, it is simple to calculate the under- or overpressure experienced by an inclusion along the PT path the host follows. By focussing diagrammatically on V , the natural variable, rather than the normally used conjugate variable, P , we represent the consequences of inclusion for arbitrary PT paths, giving the under- or overpressure as well as the equilibrium reaction progress if reaction in the inclusion is involved.