Real space, potential energy level diagrams of electrons ("band diagrams") are useful for describing devices, i.e. structures with spatially varying electronic carrier concentration. They help us to visualize situations in semiconductor materials and devices, without the need for calculations. Diagrams with the same function can be conceived of for ions in structures and devices with ionic conductivity. Such diagrams could be especially useful for describing the dynamic behaviour of systems with both ionic and electronic conductivity, such as devices incorporating semionic materials. We show how such diagrams can be constructed and indicate how their use can add to our understanding of the behaviour of mixed conductors through qualitative descriptions of processes that occur in them. We illustrate their use for a few specific cases, such as electrochromic and light emitting devices.