Another concern, beyond that of the site symmetry in a crystal, is the possibility of coupling between asymmetric units a crystal. In ionic solids the forces that interconnect all the asymmetric units in a cell makes it imperative that these couplings be considesad if one wants to understand vibration in the solid state. Even molecular crystals can exhibit unusual splittings from such phenomena. One example occurs for coupling between excited states in crystals and this is often referred to as exciton coupling. For example, the excited states ..... and ..... of two asymmetric units in ... could interact to yield composite sum and difference net states (......and.......). In this way a single excited state of the isolated molecule gives rise to two crystal states of different energy. The student with a serious interest in solid-state spectroscopy will need to delve into this in more detail. For most readers this section serves as a caveat should they attempt to interpret spectra of solids.