Abstract: 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.