A pyramid is a structure whose visible surfaces are triangular in broad outline and converge toward the top, making the appearance roughly a pyramid in the geometric sense. The cited article says the base of a pyramid can be of any polygon shape, such as triangular or quadrilateral, and its surface lines either filled or stepped, and that a pyramid has the majority of its mass closer to the ground with less mass towards the pyramidion at the apex, owing to the gradual decrease in the cross-sectional area along the vertical axis. Buildings recorded in the atlas as pyramids are grouped under this concept.