Distribution of the stars of the main sample in the ([Fe/H], [Mg/Fe]) plane. The stars have been separated into three intervals of guiding radius: inner disc, R_g ∈ [0, 6] kpc (left); intermediate disc, R_g ∈ [6, 10] kpc; and outer disc, R_g ∈ [10, 18] kpc. The numbered circles refer to the different stages of the formation of the disc (see main text): (1) formation of the thick disc; (2) quenching of the star formation; (3) dilution of the gas in the intermediate and outer thin discs; and (4) formation of the thin disc. The red lines are schematic representations of the chemical tracks. The black arrows point to the locus populated by stars which have migrated from another section of the disc.