Distribution of the surface density Σ_XY of star clusters versus distance d_XY from the Sun projected onto the Galactic plane. The density distribution of all clusters is given in black in both panels. In the upper and lower panels we show the distributions of younger (blue: log t < 7.9, green: log t = 7.9...8.3) and older clusters (brown: log t = 8.3...9.0, red: log t > 9.0), respectively. The dotted vertical line indicates the adopted completeness limit, the dashed horizontal lines correspond to the average surface density within the completeness limit of clusters in different age groups.