Top: 2D number density of disc star particles within concentric rings with |z| < 1 kpc. The number density distribution is normalised within each ring. Bottom: mean age distribution of disc stars. The red lines depict the chemical evolutionary track of stars formed from in situ gas in each of the rings. In the case of the outer rings, the sequence is discontinuous, as there are periods when no stars form from in situ gas, especially during the spin-up and high-α disc and the bridge-intermediate-α phases. The black contours enclose the region containing 90% of the star particles in the disc.