Two major features of structure in galaxy discs: the long-term presence of spiral arms, and the continuous presence of OB stars concentrated in the arms, can be explained if a spiral density wave in the stellar population interacts with star-forming gas in the disc. With a recent method we are able to perform accurate characterization of the density wave, showing that it is not a single entity, but a system of concentric waves occupying well-defined annuli in the disc. Each wave has its pattern speed and corotation radius; the strongest wave is normally associated with a bar. Each corotation radius is marked by a change in the radial gas flow from inwards to outwards, or vice-versa. Here we go a major step further, showing that a corotation is detected just where a spiral arm crosses the corotation radius of each annulus. The arms bifurcate as they cross each boundary between the density wave annuli, and the number of the corresponding corotation detections increases accordingly, producing a two-dimensional pattern, with considerable symmetry.