Birthline with rotation (Ω_ini = 2.1 × 10^-5 s^-1, solid blue line) and birthline without rotation with three contractions (long-dashed blue lines). Some points of the birthline of Behrend & Maeder (2001) are displayed for comparison (filled squares). The short-dashed lines indicate the ZAMS of Ekström et al. (2012) without rotation (black) and the pre-MS tracks at constant mass corresponding to the contractions displayed (red). The numbers along the birthline are rotational velocities (in km s^-1) along the birthline with rotation at M = 2, 6 and 14 M_⊙. Letters along the birthline indicate the evolutionary stages described in the text (Sect. 3.1). Straight dotted lines are iso-radii of 0.1, 1, 10, 100 and 1000 R_⊙ from bottom left to top right.