Schematic diagram of the birthline. The dots are observations of T Tauri stars (Cohen & Kuhi 1979, triangle), Herbig Ae/Be stars (Alecian et al. 2013, solid squares), and young O stars (Martins et al. 2012, open squares). Error bars are indicated when available. The black solid line is the ZAMS of Ekström et al. (2012), and the grey straight lines are iso-radius of indicated radii. The observational birthline (upper envelope of the observation) is illustrated in green. Red and orange arrows represent schematic tracks of intermediate-mass (red) and massive (orange) stars.