When the aim is a qualitative examination of thermodynamic phenomena and processes of two independent variable systems, the various state diagrams (p vs. v; T vs. s; i vs. s; p vs. i, etc.) are very useful. Every one of these diagrams is suitable to put in evidence some topics, whereas other ones can not be displayed. That is unavoidable since all topics can be displayed only by means of a three-dimension representation. The several diagrams, even if very different among them, have some common properties. The main aim of this paper is that of examining one property which seems interesting for the analysis of such diagrams: the order, according to which the different processes (isobaric, isothermal, isentropic, isenthalpic, isochoric) depart from a given point, is invariable, even if this order is different in the various zones of the diagrams. In addition, other properties of some diagrams are considered.