Abstract: A substantially rectangular building block having means, for use in interlocking the block to other blocks, in at least one and normally both end walls and in the center of the block. The interlocking means comprise tapered dovetail slots in the end walls, extending over the height of the end walls, and a double dovetail-shaped, tapered through opening in the center of the block. The invention also covers a wall built from these blocks using locking pins which cooperate with the interlocking means.