Notationwhich arrow? The confusion over irreversibility can be exacerbated by confusion over the meaning of different arrow notation. Equilibrium ( ) arrows indicate a reversible process obeying microscopic reversibility (𝐾 𝑒𝑞 = 𝑘 + 𝑘 - , e.g., Fig- ). Simple arrows ( ) indicate that the processes are not bound by microscopic reversibility. These are appropriate when indicating a photochemical exchange (see below) or if the arrows represent a composite process; it can be useful to combine multiple pathways linking species over a single arrow (Figure ) to simplify the depiction of complex systems. However, this can cause confusion when used incautiously. 42 "Transient". Transient means that a property of a system only persists for a defined period. Any ongoing spontaneous process may result in a transient change (e.g., if a reaction intermediate is strongly colored) that will persist until the reaction is complete (e.g., the intermediate is consumed). More specifically, it is often used in the context of supramolecular materials whose formation is associated with an ongoing chemical reaction; the material forms and then disassembles once the reaction process is complete. Transient thus has a real physical meaning it is an observable property of a system. However, transience is not in itself evidence of a non-equilibrium steady state.