and (1710). Of these the (500) is usually identified with the of chiral models. The decays and production of (1710) give strong evidence that it is also a meson. Glueball candidate The (1370) and (1500) cannot both be a quark model meson, because one is supernumerary. The production of the higher mass state in two photon reactions such as or reactions is highly suppressed. The decays also give some evidence that one of these could be a glueball. Tetraquark candidate The (980) has been identified by some authors as a tetraquark meson, along with the = 1 states (980) and (800). Two long-lived (narrow in the jargon of particle spectroscopy) states: the scalar (0) state (2317) and the vector (1) meson (2460), observed at CLEO and BaBar, have also been tentatively identified as tetraquark states. However, for these, other explanations are possible. 2 states Two isoscalar states are definitely identified: (1270) and the ′(1525). No other states have been consistently identified by all experiments. Hence it is difficult to say more about these states. 1 and other states The two isovector exotics 1(1400) and 1(1600) seem to be well established experimentally. A recent coupled-channel analysis has shown these states, which were initially considered separate, are consistent with a single pole. A second exotic state is disfavored. The assignment of these states as hybrids is favored. Lattice QCD calculations show the lightest with 1 quantum numbers has strong overlap with operators featuring gluonic construction. The (1800) 0, (1900) 1 and the (1870) 2 are fairly well identified states, which have been tentatively identified as hybrids by some authors. If this identification is correct, then it is a remarkable agreement with lattice computations, which place several hybrids in this range of masses. See also Quark model, mesons, baryons, quarks, and gluons Exotic hadrons and exotic baryons Quantum chromodynamics, flavor, and the QCD vacuum GlueX, an experiment which will explore the spectrum of glueballs and exotic mesons References Further reading Mesons Hypothetical composite particles