Carbon, nitrogen and a-element abundances determine the formation sequence of the Galactic thick and thin discs.

Using the DR12 public release of APOGEE data, we show that the thin and thick discs separate very well in the space defined by [a/Fe], [Fe/H] and [C/N]. Thick disc giants have both higher [C/N] and higher [a/Fe] than do thin disc stars with similar [Fe/H]. We deduce that the thick disc is composed of lower mass stars than the thin disc. Considering the fact that at a given metallicity there is a one-to-one relation between stellar mass and age, we are then able to infer the chronology of disc formation. Both the thick and the thin discs - defined by [a/Fe] - converge in their dependence on [C/N] and [C+N/Fe] at [Fe/H]-0.7. We conclude that (1) the majority of thick disc stars formed earlier than did the thin disc stars, (2) the formation histories of the thin and thick discs diverged early on, even when the [Fe/H] abundances are similar and (3) that the star formation rate in the thin disc has been lower than in the thick disc, at all metallicities. Although these general conclusions remain robust, we also show that current stellar evolution models cannot reproduce the observed C/N ratios for thick disc stars. Unexpectedly, reduced or inhibited canonical extra mixing is very common in field stars. While subject to abundance calibration zero-point uncertainties, this implies a strong dependence of non-canonical extra mixing along the red giant branch on the initial composition of the star and in particular on the a-elemental abundance. [ABSTRACT FROM AUTHOR]