Abstract: A building block for use in building a structure consisting of linear or stepped courses of a plurality of blocks is designed such that any two types are required, a full block and a half block. Each full or half block consists of a substantially parallel piped body having upper and lower faces, opposed side walls and opposed end walls. The side walls and the end walls define a central cavity and the blocks are arranged to receive similar blocks in overlying and underlying relationship. The outer surfaces of the side walls and end walls each include interlocking means, preferably in the form of fluting, any of which may be interengaged with any of the interlocking means of the outer surfaces of another building block of the same type. In a structure the blocks may interlock with a juxtaposed block in side to side, end to end, or end to side disposition.