Published online by Cambridge University Press: 04 August 2017
Both inner rings and outer rings are frequently observed in disk galaxies (de Vaucouleurs, 1963 and 1975). As an example, Fig. 1 shows NGC 2217 in which both occur. Further examples, e.g. NGC 2859 and NGC 3081, can be found in the Hubble Atlas (Sandage, 1961), the Revised Shapley Ames Catalogue (Sandage and Tammann, 1981), and in the morphological study of nearby barred spirals by Kormendy (1979). In this talk I will concentrate on some theoretical questions concerning rings, and in particular on their formation.