Formal thought means the appearance of new operational schemata which are based on the structures of the lattice (see Appendix A to this chapter) and a group of four transformations (I, N, Rand C) known as the Klein-four group. At the same time, the cognitive structures and specific operations of the earlier period such as seriation, forming correspondences, manipulating classes, and so on, are seen as providing the basis for the structuration appearing in adolescence.