Plot of the observed arm structure of the Galaxy in the solar neighbourhood. The Sun is at the center of the figure, and the distances on the two axes are relative to the Sun. The Galactic center is downward and outside the figure. Different kinds of objects have been added to better trace the structure, all of them shown with a same symbol (gray dots): Cepheids, open clusters, CS sources (see text for details). Curve a, in brown, represents a stellar orbit at the 4:1 resonance, and curve b an orbit at the 6:1 resonance. Curve c is not identified in terms of resonances, since it is a linear fit to the observed points. The other lines (two in red, one in green) are logarithmic spirals fitted to the data.