An extensive spectrum of light non-strange qq states up to a mass of 2400 MeV has emerged from Crystal Barrel and PS172 data onpp → Resonance → A + B in 17 final states. These data are reviewed with detailed comments on the status of each resonance. For I = 0, C = +1, the spectrum is complete and very secure.Six 'extra' states are identified. Four of them have I = 0, C = +1 and spinparities predicted for glueballs. Their mass ratios agree closely with predictions from Lattice QCD calculations. However, branching ratios for decays are not flavour blind and require mixing with neighbouring qq states. This mixing remains controversial.

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The mechanism by which a threshold may capture a resonance is examined. It involves a threshold cusp interfering constructively with either or both (i) a resonance produced via confinement, (ii) attractive t-and u-channel exchanges. The f 0 (980), X(3872) and Z(4430) are studied in detail. The f 0 (980) provides a valuable model of the locking mechanism. The X(3872) is too narrow to be fitted by a cusp, and requires either a resonance or virtual state. The Z(4430) can be fitted as a resonance but also can be fitted successfully by a cusp with no nearby resonant pole.

A combined fit is presented to data onpp annihilation in flight to final states ηπ 0 π 0 , π 0 π 0 , ηη, ηη and π − π + . The emphasis lies in improving an earlier study of ηπ 0 π 0 by fitting data at ninep momenta simultaneously and with parameters consistent with the two-body channels. There is evidence for all of the I = 0, C = +1qq states expected in this mass range. New resonances are reported with masses and widths (M, Γ) as follows: J PC = 4 −+ (2328 ± 38, Γ = 240 ± 90) MeV, 1 ++ (1971 ± 15, 240 ± 45) MeV, 0 −+ (2285 ± 20, 325 ± 30) MeV, and 0 −+ (2010 +35 −60 , 270 ± 60) MeV. Errors on the masses and widths of other resonances are also reduced substantially. All states lie close to parallel straight line trajectories of excitation number v. mass squared.