Figure 2.

The 2-, 3-, 4-, and 6-fold screw axes. For each screw, the relative translation $\tau =\frac{n}{N}$ along the z axis after the rotation is assigned by an arrow. For the neutral axes a positive or negative rotation about the z-axis (e.g., rotation by $\pm \pi$ in case of the $2_1$ screw rotation) yields the same result.