The symmetry generated by the combined operation of changing charge conjugation (C) and parity (P). CP violation occurs in weak interactions in kaon decay and also in B-mesons. CP violation occurs because of the existence of three families of elementary particles with massive particles. Since there are three types of neutrino and these neutrinos are not massless, it is thought that CP violation should occur for neutrinos. This has not been detected yet but there are currently experiments which hope to do so. See also time reversal.