to be of class ${\mathcal{G}_2}$ or ${\mathcal{G}_6}$. One can decide between these two possibilities by parallel transporting $C_2D_2E_2$ to $C_0D_0E_0$. If the angle of rotation between these triangles is null, then the isometry linking $C_0$ to $C_2$ is a translation, and the Universe has topology of class ${\mathcal{G}_2}$. On the other hand, if the angle of rotation is $\pi$, the Universe has topology of class ${\mathcal{G}_6}$. In the former case one can proceed as before and determine the length $|a|$ and the position of the axis of the screw motion. However, since for the class ${\mathcal{G}_2}$, the vector $c$ is a free parameter, its modulus and direction remain undetermined.

If the topology of the Universe turns out to be of class ${\mathcal{G}_6}$, the multiple images can be fitted within the two inequivalent configurations described in Section \[ModBuild\]. One can decide between both configurations by just looking at the directions of the axes of rotation, for if they are orthogonal the first configuration would be the correct one, while if they are parallel the correct one is the second. Using elementary geometry one can completely determine the three axes of rotation and translations (thus determining the global shape of space) if 