Distributions of stellar rotation rates at ~150 Myr (left panel) and at ~550 Myr (right panel). The distribution at ~150 Myr is a combination of measurements in the Pleiades, M 50, M 35, and NGC 2516 and the distribution at ~550 Myr is a combination of measurements from M 37 and Praesepe, as discussed in the text. Histograms showing the distributions of rotation periods in different mass bins for these two sets of stars are shown in Fig. 3.