An archetypal example of multi-step nucleation is two-step nucleation, widely described as a process where crystalline order emerges in dense liquid precursors formed during the first phase transformation in solution. Figure B shows how a two-step pathway deviates from a single-step (one-step) pathway on a two-dimensional reaction coordinate defining cluster size and order. It is assumed that the rate determining step along a two-step pathway is the emergence of order within dense liquid domains (see the snapshot in Figure ), which form readily when the supersaturation of the parent phase is sufficiently high. In this case, there are often two bottlenecks to crystallisation; however, it may be that a single energy barrier is involved in the formation of crystals from solution and that under certain conditions, traversing the lowest energy pathway leads to amorphous solute cluster intermediates. In other words, multi-step nucleation does not necessarily refer to a cascade of single-step nucleation steps.