Experimental work reported in the electrochemical literature often involves the use of different reference systems, and it is often difficult to translate between them. Reference electrodes used in solid state electrochemical systems are generally based upon the potentials of electrically neutral chemical species, and cell voltages can be calculated by the use of normal chemical thermodynamics. The identity and properties of the electrolyte play no role. In aqueous electrochemistry it is common to use reference electrodes that involve neutral species/ion equilibria at the electrochemical interface. The pH of the electrolyte is an important consideration in some cases, but not in others. These differences can be understood in terms of the Gibbs phase rule and the difference between zero-degree-of-freedom (ZDF) electrodes and those in which an additional intensive parameter, such as the electrolyte pH, must also be specified. The interrelationship between these two fundamentally different types can be readily seen using potential-pH plots, or Pourbaix diagrams.