Abstract
Twenty years ago, with the discovery of the X(3872) state in 2003, a new era in the study of hadronic spectroscopy was inaugurated. Since then, the BES, BABAR, BELLE, ATLAS, ALICE, CMS, and LHCb collaborations have observed dozens of mesonic states containing heavy quarks, which cannot be understood as conventional states formed by a quark and an antiquark. These states are now known as e…