To study the mechanism of formation and nature of MMBs, we have performed systematic theoretical work on group-15 and -16 elements, the simplest materials undergoing a change from the pre-MMB to the MMB scenario by the effect of pressure or the change in composition by heavier elements. We have used some bond descriptors, previously used by researchers defending either the metavalent or multicenter models of the bond in PCMs, as well as the bond and band pictures. In particular, we have deeply studied how pressure decreases the octahedral distortion of the trigonal R-3m (A7) and trigonal P3121 (A8) crystalline structures of group-15 and -16 elements at RP, respectively, and the Ah (sc) and Ai phases of α-Po and β-Po, respectively. As a result, we conclude that: