  Ordering of the two incommensurate charge density waves (CDW), $\mathbf{q_1}$
= (0.0, 0.243, 0.0) and $\mathbf{q_2}$ = (0.5, 0.263,0.5) in the
quasi-one-dimensional NbSe$_3$ structure is studied by means of low temperature
scanning tunneling microscopy. Larger (100) van der Waals surfaces are analyzed
using one-dimensional Fourier transforms along the three types of trigonal
prismatic columns, running parallel to the monoclinic $\mathbf{b_0}$ direction.
The procedure enables unambiguous differentiation between both CDWs modulating
individual columns. In addition, it allows a quantitative comparison of
modulation amplitudes along different columns of the same type and, in case of
sufficiently large images, also along individual columns. It is confirmed that
each CDW modulates both type-III and type-I columns with the narrowest
isosceles bases. As a consequence beating is observed between the two slightly
different modes. The bridging type-II columns appear not to be modulated. On a
large scale the modulation amplitudes along individual columns vary, suggesting
formation of CDW nanodomains. The possibility of interchanging both CDWs along
columns forming symmetry-related pairs results in a charge difference, which
might be the possible origin of CDW sliding.
