It is also possible to categorize CDWs by the mechanism that drives it. Specifically, the coupling strength \(g_q\) will determine the nature of the perturbation. At the weak coupling limit, the electron-phonon interaction softens the phonon mode around \(q=2k_F\), that is to say, reduces the energy of that phonon mode to zero. This change in the phonon dispersion comes together with a change in the electron dispersion in the same area, which opens up the CDW bandgap around the Fermi surface. Thus, this state has a well-defined momentum and also a well-defined coherence length; but not a well-defined lattice spacing. However, at the strong coupling limit, the perturbation is strong enough to completely restructure the electron bands, and offset them. As a result, this perturbation is delocalized in k-space. Consecutively, it does not have a well-defined wavevector or a coherence length, but it has a well-defined lattice spacing.