Details on how located layer lines are used to choose a selection rule which best fits the data. Based on this selection rule the filament is then reboxed and restraightened using the original digitized image so that the final image contains an integral number of helical repeats. The selection rule define helical lattices which relate the layer- line number l to the order of the Bessel function, n, contributing to the layer-line. (n is the start number, ie number around the circumference, of the contributing helix). The diffraction pattern from a helix consists not of discrete spots but of difraction spots which have been broadened into layer-lines. The order of the Bessel functions allowed to contribute to the diffraction pattern of a helix on a given layer line versus the layer line along the ordinate gives a function which is described by a lattice. Such a plot is analogous to the diffraction pattern from a planar array corresponding to a flattened helix and is called an (n,l) plot. e.g. the n,l plot corresponding to the selection rule l = 5n + 12m where m is an integer and indicates e.g. 12 (ribosomes) per repeat five turns long, whilst the n,l plot corresponding to the selection rule l = 5n + 17m indicates 17(ribosomes) per repeat five turns along.