Probability density function (PDF) of contact systems formed in Case-A binaries with M_1,  i = 7.9 − 20.8 M_⊙ as a function of the mass ratio q at the onset of contact. Data points (filled squares) show the observed mass ratio and primary mass (right axis) for all observed MW, LMC and SMC contact and near-contact systems compiled in Menon et al. (2021), including the uncertainties on their values. Systems without reported uncertainties on q are indicated with a dash symbol, and those without reported uncertainties on q and M₁ are indicated with a star symbol.