= \frac{\sum_{k>0} k S_{\perp}(k,t)}
                   {\sum_{k>0} S_{\perp}(k,t)}.$$

In Eq. (6) $k$ denotes one wave–vector component, see Eq. (4), while in Eq. (7) $k$ is defined as in Eq. (5). Clearly, $S_{\parallel}(k,t)$ is determined by structural modulations due to high–energy walls, which are reflected, for example, by sign–changes of $\psi_1$ when going along the $x$–axis, see Fig. 2. The quantity $k_{\parallel}$ therefore characterizes the inverse distance between such walls in one symmetry direction. On the other hand, $S_{\perp}$ is sensitive to the two-dimensional network of low–energy walls, and $k_{\perp}$ gives the inverse distance between such walls, cf. Fig. 3.

Curves displayed in all three parts of Figure 5 are strikingly similar in form, confirming that relaxation processes are indeed dominated by ordered domains growing in size. Moreover, $\Delta E$, $k_{\parallel}$ and $k_{\perp}$, all start to drop below their plateau–values at the same moment after the quench. This fact corroborates our linking the plateaus in $\Delta E(t)$ to the temperature-dependent incubation time for the formation of overcritical ordered nuclei. In accord with the spatial patterns shown in Figs. 2a, 3a and 3b, where high–energy walls are much further apart from each other than the low-energy ones, $k_{\parallel}(t)$ is in most cases considerably smaller 