Abstract <p>We use ternary systems as an example to discuss the need for a clear identification of invariant phase transformations. It is becoming more and more challenging to distinguish clearly and unambiguously among four-phase reactions, for example, in cases of polymorphism of reagents, decomposition or formation of compounds, and liquid–liquid immiscibility, as opposed to the currently used schematic and incomplete classification into eutectic, peritectic, all other reactions. A nomenclature of phase transformations (exemplified for isobaric phase diagrams of ternary systems) is proposed to help to differentiate such transformations by the type of phase reactions, better understand the processes occurring in the system, and more clearly present the results of experimental investigations of the system. The results can be used to design phase diagrams of ternary systems that can be calculated thermodynamically, studied experimentally, and prepared for computer modeling.</p>