We have seen that the majority of the known PSs are polyanionic thiophosphates with low P/S ratios (1/3 and below). Although a combination of visible band gaps and low effective masses can be found in some cases (Fig. and Fig. ), many thiophosphates exhibit relatively flat bands and high effective masses. Disperse bands and low effective masses seem to be more common in PSs with higher phosphorus content and with separate P and S anions, rather than polyanions. For example, a significant fraction of the known compounds with P/S = 1 have low effective masses (Fig. ). While this is a promising trend, three questions remain open for this class of PSs. 1) Are low effective masses common simply because compounds with P/S = 1 tend to have narrow band gaps? 2) Are compounds with P/S = 1 only thermodynamically stable when the metal belongs to either the d-block or the f-block? 3) Are there really so few quaternary compounds with P/S = 1 as Fig. seems to suggest?