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The definition of objects in this Category is that they must consist of four or more galaxies which, by their apparent magnitude and spacing, appear to be associated but not aligned. Just as the previous Category 17 ended with chains not so well aligned, the present Category 18 starts with groups slightly aligned and proceeds into complete non-alignment. Still, many of the Category 18 groups are also fairly good chains, if one just concentrates on the brightest galaxies or the majority of galaxies. For example, page 18.5 shows a group, AM 0007-571, which is a good alignment of bright galaxies within a broader group of fainter galaxies. A number of ring galaxies, interacting galaxies, and peculiar galaxies are included in many of these groups. On page 18.7 is shown AM 0021-571 which was also used to illustrate Category 17: Chains (page 17.21); this again emphasizes the existence of a grey area in such a classification scheme.

Towards the end of Category 18 (page 18.9) we find smaller, apparently more distant groups. One of these is AM 0232-335, which has a plume from one of the galaxies, and probably could also be considered for one of the interacting categories. Finally, on the page 18.10, we find a group containing low-surface-brightness galaxies.