Perovskite, CaTiO3, is the prototype of an extensive group of materials. They are capable of considerable chemical modification, with the further capability of undergoing structural modification by the intercalation of thin sheets of intrusive materials (both inorganic and organic) between the cubic perovskite layers, to form a range of "layered" perovskites. These changes bring about alterations in their electronic, structural, and other properties, permitting some "tuning" toward specific ends. This paper collects the limited known thermodynamic data for layered perovskites of various chemical compositions and demonstrates by example that the thermodynamic layer values are substantially additive. This additivity may be exploited by summing properties of the constituent oxides, by adding differences between adjacent compositions within a series, or even by substitution of oxides for one another, thus permitting prediction beyond the known range of compositions. Strict additivity implies full reversibility so that the additive product may be unstable and may undergo structural changes, producing materials with new and potentially useful properties such as ferroelectricity, polarity, giant magnetoresistance, and superconductivity.