Abstract: The invention provides a building block having a shape derived from that of a cuboid, wherein the upper corners of the front face (1) are cut-away and a recess communicates between the bottom face (2) and the rear face (3). Preferably, the sum of the depth to which the said corners are cut away backwards from the front face and the depth to which the recess extends forward from the back face is greater than the depth of the block. Thus, when the blocks are built into a wall in the conventional manner louvres are formed by the recess of one block and the adjacent cut-away portions of two blocks below. The cut-away portions at the corners may be tetrahedral or half segments of a circle or ellipse, for example. The recesses may be tetrahedral or segments of a circle or ellipse.