line repetition groups

The possible symmetries of arrays extending in one direction with a fixed repeating distance. Linear polymer chains in the crystalline state must belong to one of the line repetition groups. Permitted symmetry elements are: the identity operation (symbol l); the translation along the chain axis (symbol t); the mirror plane orthogonal to the chain axis (symbol $m$ ) and that containing the chain axis (symbol $d$ ); the glide plane containing the chain axis (symbol $c$ ); the inversion centre, placed on the chain axis (symbol $i$ ); the two-fold axis orthogonal to the chain axis (symbol 2); the helical, or screw, symmetry where the axis of the helix coincides with the chain axis. In the latter case, the symbol is $s A * M / N$ , where $s$ stands for the screw axis, $A$ is the class of the helix, * and / are separators, and $M$ is the integral number of residues contained in $N$ turns, corresponding to the identity period ( $M$ and $N$ must be prime to each other). The class index $A$ may be dropped if deemed unnecessary, so that the helix may also be simply denoted as $s M / N$ .

Source:

Purple Book, p. 79