  Measurements performed with pairs of charm mesons produced at threshold from
the decay of the $\psi(3770)$ resonance are of great value in flavour physics.
The quantum correlation that exists between the two mesons allows unique access
to strong-phase information, which is essential input to flavour-physics
studies conducted in other environments. An excellent example from the BESIII
collaboration is a recent determination of the strong-phase difference between
$D^0$ and $\bar{D}^0$ mesons in the decay $D^0 \to K^0_S\pi^+\pi^-$, which has
enabled recent measurements to be performed of the $C\!P$-violating phase
$\gamma$ and $D^0-\bar{D}^0$ oscillations by the LHCb experiment at CERN. These
$\psi(3770)$ data, and also those collected just above the thresholds for
$D_s^+$ and $\Lambda_c^+$ production, can also be exploited in many other ways
that are of benefit to flavour-physics studies. These synergies are reviewed,
and the need for larger threshold data samples in the near future is
emphasised.

High Energy Physics - Experiment;High Energy Physics - Phenomenology