Scheme illustrating the different sequence of L and L′ along the a axis as a function of the modulation vector q = α(a′)* and t0. The sequence can be deduced from the intersections of a black solid line with black (L′) and grey (L) (orange in the electronic version of the journal) domains. The L domain is defined by the central point x4 = 0 and width Δ = 0.5. L and L′ domains are related by the unit-cell centring X = 00. The coefficient α in the q vector expression is equal to the tangent of the angle between the black solid line and the x1 axis.