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The spectrum of hadrons is important for understanding the confinement ofquantum chromodynamics. Many new puzzles arose since 2003 due to the abundanceof experimental discoveries with the $XYZ$ structures in the heavy quarkoniummass region being the outstanding examples. Hadronic resonances correspond topoles of the $S$-matrix, which has other types of singularities such as thetriangle singularity due to the simultaneous on-shellness of three intermediateparticles. Here we briefly discuss a few possible manifestations of trianglesingularities in the $XYZ$ physics, paying particular attention to theformalism that can be used to analyze the data for charged $Z_c$ structures inthe $\psi\pi$ distributions of the reaction $e^+e^-\to \psi\pi^+\pi^-$.

Triangle Singularities and Charmonium-like $XYZ$ States