Probability distribution functions of merger properties for BL and BD galaxies. Panels a–d show the distributions of stellar mass ratio, |cos θ_orb|, gas mass fraction, and cos θ_spin, respectively, for major mergers (as defined in Section 2.2). Solid and dashed horizontal lines indicate medians and interquartile ranges. Grey hatched regions mark classification thresholds: coplanar mergers have |cos(θ_orb)| > 0.7; perpendicular ones have |cos(θ_orb)| ≤ 0.3; corotating mergers satisfy cos(θ_spin) > 0.7, perpendicular mergers fall in −0.15 < cos(θ_spin) < 0.15, and counterrotating mergers have cos(θ_spin) <  − 0.7.