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Occasionally spiral galaxies are encountered which have three well-defined arms instead of the usual two. The kind of perturbation which would create such arms is difficult to hypothesize so this Category has been assembled as a challenge to theories of spiral structure. The clearest examples are often more distant objects where resolution does not give details and hence emphasizes the general form of the arms. On page 11.1 six examples are given. It should be noted that AM 2035-521 is also illustrated at a larger scale as the prototype of Category 23: Close Pairs. On page 11.2 the spiral AM 0915-221 shows doubling in each of its three arms, but the western arm is particularly striking, giving rise to the appearance of a four-armed spiral. On page 11.3 the spirals are still basically three-armed but with some detailed structure. Object AM 0957-265 is noteworthy because the third arm is so much brighter and thinner than the other two. The stellar image near the end of this arm is a foreground star.

On page 11.4 three objects are shown in which the arm coming off the end of a bar is double. These objects are closely connected with some of the objects in Category 9: M51-Types (e.g., pages 9.10 and 9.11) which show longer, more parallel, double arms coming off the ends of bars. Also on page 11.4 are illustrated three objects where the third arm does not originate from the nucleus but starts from a point somewhere along one of the two arms. Page 11.5 shows a nearby spiral AM 0131-294 photographed with the CTIO 4m telesope; it illustrates either a four- or five-armed spiral, depending on interpretation, and thereby introduces the transition to multi-armed spirals. On page 11.6 two objects AM 0340-472 and AM 0242-600 have been shown before, in the beginning of Category 8, because they have small apparent companions, but they are also examples of multi-armed spirals.

Page 11.8 shows a photograph of the nearby spiral AM 0052-375 (NGC 300) in the Sculptor Group of galaxies. Because it is nearby it is considerably resolved. It if were more distant two interesting aspects would be more evident. One is that it is a transition between a two-armed spiral and a multi-armed spiral, hence its inclusion at this place in Category 11. At greater distance it also would appear to have condensations (here seen as massive associations) on the ends of each of its two major arms. This might also relate it weakly to Category 9: M51-Types.

On page 11.9 is shown a galaxy, AM 0545-341, that has a high-surface-brightness inner disk and low-surface-brightness outer arms. This is included as an example of the transition from grand-design, multi-armed spirals to more chaotic spirals. It is also related to the peculiar spirals in Category 12 which have very low-surface-brightness exterior regions surrounding a bright disk (pages 12.4 and 12.5).

The final illustration in this Category, page 11.10, shows an assortment of multi-armed spirals. One of these, AM 0102-274, shows arms that almost give an impression of being interlaced.