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We develop the spectroscopy of $c\bar c c\bar c$ and other all-heavytetraquark states in the dynamical diquark model. In the most minimal form ofthe model (e.g., each diquark appears only in the color-triplet combination;the non-orbital spin couplings connect only quarks within each diquark), thespectroscopy is extremely simple. Namely, the $S$-wave multiplets containprecisely 3 degenerate states ($0^{++}$, $1^{+-}$, $2^{++}$) and the 7 $P$-wavestates satisfy an equal-spacing rule when the tensor coupling is negligible.When comparing numerically to the recent LHCb results, we find the bestinterpretation is assigning $X(6900)$ to the $2S$ multiplet, while a lowerstate suggested at about $6740$ MeV fits well with the members of the $1P$multiplet. We also predict the location of other multiplets ($1S$, $1D$, etc.)and discuss the significance of the $cc$ open-flavor threshold.

The Simple Spectrum of $c\bar c c\bar c$ States in the Dynamical Diquark Model