Fig. shows the cumulative κ iso at T = 300 K for the four representative systems in Fig. , viz. Pnma GeSe, R3m GeTe, Cmcm SnSe and Fm 3m SnTe. For Pnma GeSe, which has a bandgap in its dispersion (c.f. Fig. ), around 85% of the thermal conductivity is due to transport through the 50% of the modes below the gap. There is also a clear flattening of the cumulative κ iso over the gap region, as the absence of modes means there is no thermal transport over this frequency range. The cumulative κ iso of R3m GeTe shows similar behaviour, with a notable reduction in the rate of accumulation around ca. 3 THz, coinciding with the dip in the DoS (c.f. Fig. ) and a ∼75-80% contribution from the modes below this frequency. While Cmcm SnSe does not have a phonon bandgap, if we take the midpoint of the spectrum to be ∼3 THz, around 85% of the κ iso is from transport through modes with frequencies below this cutoff, which is consistent with the Pnma and R3m systems. On the other hand, for the heaviest compound, Fm 3m SnTe, for which the phonon spectrum extends to around 4 THz, around 95% of the thermal conductivity is through modes below ∼3 THz. Across all four compounds, therefore, the low-frequency phonon modes with f λ < 2.5-3 THz contribute > 75% of κ iso . This can be understood based on the fact that the lower-frequency modes tend to have wider dispersions than the higher-frequency modes, resulting in larger group velocities, and are also more heavily populated, i.e. have larger occupation numbers n λ at a given temperature (c.f. Fig. and Eq. 4/5).