Proper motion histogram of stars in the central fields of the Quintuplet (left) and Arches (right) clusters. The motion parallel to the Galactic plane (black) shows a pronounced tail of field sources, as indicated in the proper motion diagram (Figs. 7 and 8), while the motion perpendicular to the plane (red histograms) is dominated by the velocity dispersion. The Gaussian fit to the latitude motion (red solid lines) in Field 1 provides the 3σ criterion for selecting cluster members in both central fields. In the case of the Arches cluster, the same membership criterion is imposed in all outer fields.