  We revisit the basic properties of Fano-Feshbach resonances in two-body
systems with van der Waals tail interactions, such as ultracold neutral atoms.
Using a two-channel model and two different methods, we investigate the
relationship between the width and shift of the resonances and their dependence
on the low-energy parameters of the system. Unlike what was previously believed
[Rev. Mod. Phys. 82, 1225 (2010)] for magnetic resonances, we find that the
ratio between the width and the shift of a resonance does not depend only on
the background scattering length, but also on a closed-channel scattering
length. We obtain different limits corresponding to different cases of optical
and magnetic resonances. Although the generalisation of the theory to the
multi-channel case remains to be done, we found that our two-channel
predictions are verified for a specific resonance of lithium-6.
