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 $\text{s}(A\ast M/N)$, where $\text{s}$ 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 $\text{s}\left(M/N\right)$.