Set of rules proposed in 1975 to classify photochemical reactions based on correlation diagrams and under the assumption that all photochemical processes are controlled by the generation of primary products with characteristics of diradicals, i.e., considering the physical fact that the initial single excitation separates two electrons of a pair into two distinct orbitals. The reactions are classified according to their topicity, i.e., the total number and nature of available radical sites generated in the primary photochemical product.