This paper is divided into three sections. In section 2, we analyze the effect of pressure on the structural, vibrational, and electronic properties of the crystalline structures of pnictogens (As, Sb, Bi) and chalcogens (Se, Te, Po) using density-functional theory (DFT) calculations, although our results can be extended to the rest of group-15 and -16 elements. Using theoretical bonding descriptors previously used by the proponents of the two current models of MMBs in PCMs, we confirm the MMB formation in these two elemental families as the octahedrally-coordinated crystalline structures are approached at HP, thus showing that pnictogens and chalcongens at HP can help us to understand the nature of the MMB in PCMs. The only exception in our analysis is Po, whose crystalline α and β structures already exhibit MMBs at RP. We will also show that the existence of MMBs in these elements and PCMs is consistent with the most recent Wuttig's 2D map showing ES vs. ET ; in other words, the ES value and the ES vs. ET map are reasonably good tools for understanding bonds, like MMBs, even in complex materials, such as PCMs, with different kinds of bonds in a given atomic structure. In section 3, we describe the mechanism of the pressure-induced transformation from the pre-MMB scenario to the MMB scenario in pnictogens and chalcogens using both the real-space (bond) and the reciprocal-space (band) pictures. We will show that the mechanism of MMB formation is similar for pnictogens and chalcogens and proceeds through several stages, which are different for both elemental families. This universal mechanism is expected to apply also to PCMs. In section 4, we address the MMB nature and show that the mechanism of MMB formation in pnictogens and chalcogens (and consequently in PCMs) is similar to that of EDMB formation in diborane and in atomic/polymeric nitrogen and hydrogen phases at HP. This result agrees with the multicenter character previously proposed for MMBs , but not with the previous hypotheses that metavalent bonding is a new type of bond and that MMBs are ERMBs . Our results in fact show that the mechanism of MMB formation is completely different from that of ERMB formation; a result that agrees with Wuttig's claims . Consequently, we propose that MMBs in solids, such as PCMs and pnictogens and chalcogens at HP, are equivalent to EDMBs that are formed in solids, e.g. in Zintl phases, intermetallics, and cluster compounds. Furthermore, we propose the geometries of the hypercoordinated units with EDMBs in solids formed by electronrich elements, such as group-15 and -16 elements, along one, two, and three dimensions. Details of the ab initio simulations for most of the materials studied in this work are given in section 1 of the Electronic Supplementary Information (ESI).