A block combinable with other similar blocks to form a wall comprises a front surface; a top surface including a plurality of bosses, two or more of which are located adjacent each other in a direction along the front surface; and a bottom surface including a groove that has a floor. Each of the two or more bosses of the top surface includes a summit, and the summit of each define a plane. When the block is placed on another similar block such that the bottom surface of the block and a top surface of the other similar block oppose each other to form an interface region, the groove holds two or more bosses of the other similar block that are included in the other similar block's top surface, and the floor does not intersect the plane defined by the two or more bosses of the other similar block.