Upper panel: effects on the chemical evolution of the solar neighbourhood in the [α/Fe] versus [Fe/H] abundance ratios of different values for the delay of the beginning of the second infall (the quantity t_max in Eq. (4)). Lower panel: [α/Fe] abundance ratios as a function of [Fe/H] predicted by our two-infall chemical evolution model (salmon circles) taking into account the average observational errors on age and metallicity (following Eqs. (10) and (11)) for stars older than 11 Gyr, considering a delay for the beginning of the second infall of t_max = 3.3 Gyr. Also plotted is the observational data, colour coded as in Fig. 1.