tokens: of SEGUE dwarfs. They measured, for the disc as a whole, a vertical metallicity gradient of dex kpc −1 , which is in agreement with our measurement. However, their low-α population has a gradient consistent with zero: d[M/H]/d dex kpc −1 . However, their intermediate α sub-population with 0.2 [α/M] 0.3 has d[M/H]/d dex kpc −1 , which agrees with our low-α metallicity gradient. We thus conclude that the discrepancy between our result and that of Schlesinger et al. ( 2014 ) is largely due to the chemical separation criteria (also see Ciucă et al. 2018 , who found an age-dependence for the thin disc vertically metallicity gradient, such that the youngest population has a flatter gradient). Mikolaitis et al. ( 2014 ) measured a slightly shallower gradient of d[M/H]/d z = −0.11 ± 0.01 for Gaia -ESO dwarfs and giants. The Gaia -ESO sample is more metal-poor overall, and Mikolaitis et al. ( 2014 ) separated thick disc stars by the location of underdensities in their [Mg i /M] histograms (their fig. 3). The dividing line is at different values of [Mg i /M] for different metallicity regimes. In fig. 10 of Mikolaitis et al. ( 2014 ), it is clear that their sample is biased against metal-rich stars, such that there are very few stars with [M/H] > 0 (also see Stonkutė et al. 2016 ). 5.2.2 The high-α population The vertical metallicity distribution of the thick-disc (high-α) stars is relatively flat compared to the low-α population, at −0.058 ± 0.003 dex kpc −1 . Several authors have measured the vertical gradient for the thick disc, using different methods to define this population. Earlier studies, such as Gilmore et al. ( 1995 ) and Prieto et al. ( 2006 ), reported no vertical metallicity gradient in the thick disc (Prieto et al. 2006 quoted an upper limit of d[M/H]/d z = 0.03 dex kpc −1 ). More recently, Boeche et al. ( 2014 ) concluded that the vertical metallicity gradient of the thick disc is consistent with zero, based on a sample of RAVE giants. However, other studies, using a combination of metallicity or kinematics to separate the thick disc, have reported a shallow metallicity gradient. Katz et al. ( 2011 ) observed sub-giants at two lines of sight: ( l, b ) = (51°, 80°) and (5°, 46°) at low resolution. Their MDFs show signs of bimodality, and the thick disc was defined as stars centred around [M/H] ≈ −0.5 dex. The vertical gradient measured by Katz et al. ( 2011 ) is −0.068 ± 0.009 dex kpc −1 , consistent with our value. Ruchti et al. ( 2011 ) observed a number of metal-poor thick disc candidates at high resolution using the MIKE, FEROS and UCLES spectrographs (λ/Δλ ≈ 35 000–45 000). They classified their stars based on a Monte Carlo simulation of space motion U, V, W , assuming Gaussian errors on the velocities and distances. By further restricting their α-enhanced sample with thick disc kinematics to metal-poor stars only ([M/H] ≤ −1.2), they avoid most thin disc contamination. The measured gradient is −0.09 ± 0.05 dex kpc −1 ,
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