  In the presence of multiple bands, well-known electronic instabilities may
acquire new complexity. While multiband superconductivity is the subject of
extensive studies, the possibility of multiband charge density waves (CDWs) has
been largely ignored so far. Here, combining energy dependent scanning
tunnelling microscopy (STM) topography with a simple model of the charge
modulations and a self-consistent calculation of the CDW gap, we find evidence
for a multiband CDW in 2H-NbSe$_2$. This CDW not only involves the opening of a
gap on the inner band around the K-point, but also on the outer band. This
leads to spatially out-of-phase charge modulations from electrons on these two
bands, which we detect through a characteristic energy dependence of the CDW
contrast in STM images.
