Abstract: A bent steel insert is provided with top and bottom pairs of abutments for engaging the inside surfaces of top and bottom rows of concrete blocks to center the rows on top of each other. The preferred embodiment is made of a single strip of steel by: bending it adjacent one end at approximately a 90.degree. angle; by doubling it back upon itself to project to the opposite side of the original plane of the strip; and then bending it back again to the original plane to provide top and bottom abutments at one end. The opposite end of the strip is similarly bent to form abutments which are a mirror a image of those previously described to thereby provide pairs of top and bottom abutments for centering building blocks on top of each other.