Abstract: A mason wall cementitious building block system comprising two lightweight dry-stackable block units and methods of using such units, including a wall unit and a corner/end unit, connected one to another in an interlocking fashion by means of male posts and female sockets, and variants on each of said units for capping the uppermost course of an assembled wall. The male posts angle rearwardly to define partial apertures, such that adjoining blocks define an aperture which extends vertically between the adjoined blocks for placement of vertical reinforcement, electrical and plumbing chase, and for the introduction of mortar or cement. The top surface defines a recessed cavity for placement of rebar when stacked. The corner/end unit of the present invention interconnects with wall units also by means of posts and sockets and may function as either an end or a corner, requiring only the removal of a small portion of the block at prescored cut lines to alter functions.