The electromagnetically bound or Coulomb-bound pion-pion system, the π⁺π⁻ atom, also called A_2π. Nowadays, this doubly exotic system is called pionium. In some older literature the term pionium was used for atoms consisting of only one pion and a muon or an electron. Eighteen pion-muon (πμ) atoms, formed in the decay K⁰_L → (πμ)_atom ν were detected in 1976 in an experiment at Brookhaven National Laboratoty in Upton, New York.