Inter- and intramolecular photochemical processes involving the addition of a C–C double (or triple) bond (i) to the 1,2-positions of an arene in which case it is called an ortho photocycloaddition, with formation of a benzocyclobutene (or a benzocyclobutadiene) derivative, (ii) to the 1,3-positions of an arene in which case it is called a meta photocycloaddition, with formation of tricyclo[3.3.0.0^2,8]oct-3-ene (or octa-3,6-diene) derivatives, or (iii) to the 1,4-positions of an arene in which case it is called a para photocycloaddition, with formation of bicyclo[2,2,2]oct-2-ene (or octa-2,5-diene) derivatives.