Left: rotation periods of late-type stars at an age of 220 Myr as a function of their initial rotation period on the ZAMS calculated using the rotation evolution model introduced by Barnes (2010). Right: Rossby number of late-type stars at an age of 220 Myr as a function of their initial rotation period on the ZAMS calculated with the same model but scaled to the calibration value of the Sun convective turnover time used by Wright et al. (2011). The horizontal line shows the critical Rossby number, which separates the saturated regime (at low Rossby number) from the non-saturated regime of X-ray emission.