Contrast and changes in sparseness and density can produce rhythmic movement. However, for the composition of elevations, such contrast is a necessary but insufficient condition for the formation of rhythm. Except when buildings are unevenly spaced in groups, other techniques need to be employed to strengthen the rhythmic change of elevations, creating different heights and layers in depth. The height difference is expressed by the varied undulations of contour lines. For example, the richly varied and rhythmic combination of the elevations in the Garden of Harmonious Charms arise from the alternating change of the directions, volumes plus the stories and forms of roofs in particular, of the structures. The above elements have a direct impact on the external contour lines of the elevations.