Comparison between the MTI (top) and HBI (bottom) linear growth rates numerically measured and theoretically computed thanks to the dispersion relations. The initial velocity perturbation is a fundamental divergence-free diagonal mode (k_y = k_z = 2π/L) with amplitude 10⁻⁴. Different colour tones are for the different Prandtl numbers. Runs with resolution 64² and 256² are represented by stars and dots.