  We discuss some general features of the effective range expansion, the
content of its parameters with respect to the nature of the pertinent
near-threshold states and the necessary modifications in the presence of
coupled channels, isospin violations and unstable constituents. As illustrative
examples, we analyse the properties of the $\chi_{c1}(3872)$ and $T_{cc}^+$
states supporting the claim that these exotic states have a predominantly
molecular nature.
