  Hybrid Improper Ferroelectricity (HIF) allows the generation of an electrical
polarization in the AA'BB'O$_6$ double perovskite materials thanks to the
combination of two non-polar octahedral distortions. Nevertheless, for selected
combination of the A/A' cations a non-polar incommensurate phase is observed
with average symmetry $C2/m$. Thanks to a detailed crystallographic description
of the incommensurate phase, based on electron, neutron and x-ray diffraction
data, we show that the incommensurate modulation is related to an abrupt change
of the out-of-phase tilting along the a- and c-axis whereas the tilting along
the b-axis remain constant across the structure. By using group theory and
symmetry analysis we show that we observe an incommensurate analog of HIF which
induces a hybrid improper dipolar density wave in NaLaCoWO$_6$. The dipolar
ordering is due also in this case to a trilinear invariant involving the
commensurate and incommensurate octahedra tilting's.
