  We consider the spontaneous formation of striped structures in a holographic
model which possesses explicit translational symmetry breaking, dual to an
ionic lattice with spatially modulated chemical potential. We focus on the
perturbative study of the marginal modes which drive the transition to a phase
exhibiting spontaneous stripes. We study the wave-vectors of the instabilities
with largest critical temperature in a wide range of backgrounds characterized
by the period and the amplitude of the chemical potential modulation.
  We report the first holographic observation of the commensurate lock-in
between the spontaneous stripes and the underlying ionic lattice, which takes
place when the amplitude of the lattice is large enough. We also observe an
incommensurate regime in which the amplitude of the lattice is finite, but the
preferred stripe wave-vector is different from that of the lattice.
