The paper unifies ideas from models of appearance and image registration. It proposes a measure that does not only characterise model quality, but is also capable of reasoning about registration. These two strands - modelling and registration - inherently work towards achieving the same goal. Registration of a group of images leads to correspondence, which then forms a model of shape and intensity. The model can be used inversely to argue about the quality of its seminal registration. The important concepts are explained in the remainder of this section.