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Galaxies in this Category are more or less equal in size and show strong signs of interaction. It should be noted that while interacting doubles are by far the most numerous of any of the systems which were catalogued, they are sparsely illustrated in this photographic volume. This is because many interacting doubles are repetitive in form. The photographs illustrate cases which are felt to be representative of the various sub-categories plus a few cases which are especially interesting.

2a. E + E Interaction

Only when both of the interacting galaxies are smooth, E-like galaxies or galaxies not classifiable as spirals, are they considered to be in this sub-category. Some, but not all, of these types are at the centres of clusters and may be surrounded by extensive envelopes reminiscent of the cD phenomenon.