a building brick having upper and lower faces . the upper face has a pair of longitudinally extending triangular ridges along each side thereof , each ridge having a narrow flat apex surface . the lower face has a pair of complementary depressions formed by a central longitudinally extending upstanding portion of height less than the height of the ridges , so that when similar bricks are engaged , their depressions and ridges interlock to align their viewable faces and restrain movement of the bricks in a transverse direction , while leaving a space at least 0 . 3cm thick and of width equal to at least 40 percent of the width of each brick , between successive rows of bricks to contain bonding material , and with the upper brick resting on the apex surfaces of the brick beneath it . a similar vertical space is provided between the end faces of the bricks , so that a wall can be assembled without mortar and can then be mortared by pouring a thin mortar into one or more of the vertical spaces .