There are three major mechanisms of heterochrony, each of which can change in either of two directions, giving six types of perturbations, which can be combined in various ways. These ultimately result in extended, shifted, or truncated development of a particular process, such as the action of a single toolkit gene, relative to the ancestral condition or to other conspecifics, depending on whether inter- or intraspecific heterochrony is the focus. Identifying which of the six perturbations is occurring is critical in identifying the actual underlying mechanism driving peramorphosis or paedomorphosis.