Effect of IMF slope on binary star fraction in a population of total mass 10⁶ M_⊙, given the stellar mass-dependent binary fractions implemented in BPASS v2.2. The lower region between the x-axis and the first pair of lines indicates the fraction of the total stellar mass for a given IMF slope α_u which is assigned to single star models, the central region the mass fraction in primary star models and the upper region the mass fraction assigned to their less massive secondary companions. Regions between each pair of lines (with hatching in two styles) switch between stellar model types depending on the IMF. Models assume a single power law break (α_m = α_l), The upper cut-off mass, M_u, is shown by line style (100 M_⊙ - solid; 300 - dot-dash). The curves for M_u =150 & 200 M_⊙ lie between these curves.