Gas-phase mass–metallicity relation for 21 star-forming galaxies at redshifts 3 < z < 3.8 from the NIRVANDELS survey, color-coded according to galaxy density (higher or lower than the sample median = 0.003 gal Mpc⁻³) and to the overdensity flag, in the upper and lower panel, respectively. The larger colored squares represent the median gas-phase metallicities and the median stellar mass of the galaxies in the same bin. The vertical and horizontal error bars represent, respectively, the 1σ uncertainty of the median metallicity and the standard deviation of stellar masses in each bin. We do not display the median point where the bin contains only one galaxy.