A building system includes a foundation block and a wall block. Each block has opposing first and second main surfaces, a first top end and an opposing second bottom end, and opposing first and second side surfaces extending between the first and second main surfaces and between the top and bottom ends. Portions of the first and second main surfaces of the foundation block proximate the top end thereof taper axially inwardly toward each other and toward an interior of the block to form a pair of inwardly tapered surfaces. The bottom end of the wall block includes a pair of outwardly tapered surfaces which extend axially outwardly away from each other and toward an exterior of the wall block. The pair of outwardly tapered surfaces of the wall block engage the pair of inwardly tapered surfaces of the foundation block in an assembled configuration to form a first vertical stack member.