SO far the formation of spiral arms, especially regular arms, has not been satisfactorily explained. The investigation of this phenomenon can be facilitated by an analysis of similar, but more peculiar, objects—barred spirals. A characteristic barred galaxy has a straight bar, which at the ends transforms abruptly into spiral arms. The population of bars and arms belongs to type I. For the bar to rotate and conserve its straight-line form it is necessary that the gravitational field within the limits of the bar corresponds to solid body rotation, as, for example, inside a homogeneous sphere. Beyond the ends of the bar rotation is differential, that is, the density should decrease sharply with distance. The density distribution is not determined by the bar but by the old stars of low luminosity. Consequently the old stars in an SB should form either a homogeneous sphere or a disk with a very small concentration toward the centre of the galaxy and their density should suddenly decrease at some distance.