Extent of the Mg-Al anti-correlation, measured as σ[Al/Mg] (top panels) and Δ[Al/Mg] (bottom panels), based on the sample described in the text. The behaviour as a function of average [Fe/H] (left panels) and total log M (present-day mass, right panels) of each GC is shown. The GCs in the left panels are coloured as a function of log M, where yellow corresponds to log  M = 4.19 dex (the lowest mass in the sample) and dark orange to log  M = 6.05 dex (the highest mass). In the right panels, points are coloured as a function of their metallicity with red corresponding to [ Fe / H ] = − 0.5 dex (the highest metallicity in the sample) and blue to [ Fe / H ] = − 2.5 dex (the lowest metallicity). Our models in the form a [ Fe / H ] + b log  M + c are also plotted as lines coloured based on mass or metallicity.