Abstract: A building block and masonry wall constructed therefrom are provided wherein the building block has a pair of spaced opposite generally parallel side walls, spaced top and bottom faces and a pair of end spaces, an intermediate generally centrally extending horizontal groove in said top face, a pair of narrower grooves, one on either side of said centrally extending groove on said top face, an intermediate vertical passage of generally keyhole shape in said block corresponding in width to the widest portion of the central grooves on said top face, a pair of vertical passages on each side of said intermediate vertical passage extending from the top narrower grooves, and at least one vertical recess in each end intermediate the side walls and having a cross section substantially equal to one half of said intermediate vertical passage.