The Woodward–Hoffmann rules can also explain bimolecular cycloaddition reactions through correlation diagrams.  A [p + q] cycloaddition brings together two components, one with p π-electrons, and the other with q π-electrons.  Cycloaddition reactions are further characterized as suprafacial (s) or antarafacial (a) with respect to each of the π components.  (See below "General formulation" for a detailed description of the generalization of WH notation to all pericyclic processes.)