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The 2020 announcement by LHCb of a narrow structure $X(6900)$ in thedi-$J/\psi$ spectrum -- a potential $c\bar c c\bar c$ state -- has opened a newera in hadronic spectroscopy. In this talk, we briefly survey theory workspreceding this event, examine key features of the observed spectrum, and thendiscuss how subsequent theory studies (including with the author's owndynamical diquark model) have interpreted these features. We conclude withproposals for experiments capable of distinguishing competing interpretations.

All Heavy Tetraquarks: The Dynamical Diquark Model and Other Approaches