The BaAl 4 (ThCr 2Si 2) tI10 structure, MN2X2, is not only the most populous of all known structure types, being adopted by some 400 phases, but is representative of a new group of metallurgically important structures that do not adhere to classifications of Cu-type close packings or tetrahedral close packings. The structure can be described by a square pyramidal framework of N and X atoms which by edge-sharing from slabs in (001) planes, the slabs being interconnected by X X bonds along  (in perhaps 10% of the phases no  X X bonds exist). This framework creates tetragonal hexagonal prismatic voids, that enclose the large M atoms. The average observed N X and  X X distances are interpreted in a model that shows that the framework, which is responsible for structural stability, accounts for almost all of the valency electrons provided by N and X. The M atoms (diameters 3.1 to 5.1 Å) are accommodated in the voids of the framework according to the principles of geometrical packing, apparently without size restriction. In some cases M N or M X interactions are shown to control the variable a, c, and zX structural parameters, but they only very modestly influence the N X distances of the framework. Some comparison with the Cu 2Sb tP6 structure is made.