Fundamentally SDWs and CDWs involve the development of a superstructure in the form of a periodic modulation in the density of the electronic spins and charges with a characteristic spatial frequency q that does not transform according to the symmetry group that describes the ionic positions. The new periodicity associated with CDWs can easily be observed using scanning tunneling microscopy or electron diffraction while the more elusive SDWs are typically observed via neutron diffraction or susceptibility measurements. If the new periodicity is a rational fraction or multiple of the lattice constant, the density wave is said to be commensurate ; otherwise the density wave is termed incommensurate.