Abstract: A mortarless, lightweight building block and walls formed therefrom. The block is generally rectangular and comprises side walls, end walls, and an interior wall which divides the interior of the block into two vertical passages through the block. In certain blocks, parts of the end and interior walls are removed to form horizontal channels through the blocks. The blocks are provided with projections and recesses having rectangular cross sections by which means they may be interlocked together to form walls. In such a wall, the vertical passages and horizontal channels may receive reinforcing structures to impart added strength and load bearing properties to the wall.