Distributions of the regressed OCs. The OC ages have been grouped into: (a) 20−40 Myr, (b) 40−60 Myr, (c) 60−80 Myr, (d) 80−100 Myr, and traced back to 20, 40, 60, and 80 million years ago, respectively. The solid curved lines trace the centres (and dotted lines the widths enclosing 90% of the masers) of the best-fit spiral arms given by Reid et al. (2019), and have been rigidly rotated back to the past for comparison. The background is a new concept map of the Milky Way (credited by: Xing-Wu Zheng & Mark Reid BeSSeL/NJU/CFA), which is also rigidly rotated back to the past. The adopted pattern speed is 27 km s⁻¹ kpc⁻¹.