The lines in Pourbaix diagrams represent redox and acid-base reactions, and are the parts of the diagram where two species can exist in equilibrium. For example, in the Pourbaix diagram for Fe below, the horizontal line between the Fe3+ and Fe2+ regions represents the reaction Fe3+(aq) + e- = Fe2+(aq), which has a standard potential of +0.77 V. While we could use standard potentials for all these lines, but in practice Pourbaix diagrams are usually plotted for lower ion concentrations (often 1 mM) that are more relevant to corrosion and electrochemical experiments.