Taking the case where k = g = 1 as our reference point, we introduce bonding inhomogeneity by increasing the difference ∆ = (kg)/k while maintaining the average (k + g)/2 = 1. The ν a is maximised when k = g (homogeneous bonding) and falls to zero at ∆ = 1 (g = 0, i.e. one of the two bonds is broken) when phonon propagation is blocked. This simple model therefore shows that irregular chemical bonding reduces the group velocity. A large primitive cell (large n a ), or, for a given n a , a spacegroup with fewer symmetry operations, implies fewer equivalent chemical bonds and therefore more irregular bonding, which explains the strong correlation with the group velocities.