Abstract: A building block system comprised of three basic blocks. A first block has the shape of a rectangular prism and is of a more or less standard cinder block design. A second block has a main body portion that is similar in size and shape to the first block. The second block also includes a pair of spaced apart, mirror image protrusions that extend outwardly from one of the vertical faces of the main body portion. The protrusions define a vertical channel that is laterally centered on the second block. A third block also has a main body portion similar in size and shape to the first block. The third block also includes a pair of spaced apart protrusions extending from one of the vertical faces of its main body portion. The protrusions on the third block are laterally offset to one end of the block. The blocks can be stacked together end-to-end and in courses to form rigid, load-bearing structures.